[GT540 Optimus General] marlboroazrag:
Yes You can. But why you want to get 2.1 back anyway 2.3 is way better :)

Now lets Start.

Follow these steps :

1.Download USB drivers for lg gt540 and 2.1 rom e.g: 2.1 fastboot,Europe Open or others from here

2.KDZ_FW_UPD_EN from HERE

3.extract the file,Install msxml.msi. and open KDZ_FW_UPD.exe

4.make sure of TYPE 3GQCT and PhoneMode to DIAG

5.Browse for ROM file that You downloaded at Step 1

6.connect your phone. Pull out SD card for sure. and activate debugging mode

7.Click Launch software update

NOTE : BackUP all your Apps and Contacts etc..

NOTE : some 2.1 Roms or maybe all of them doesn't have boot animation (Just black screen)

UPDATE log should be similar to this

19:46:25 : Launching SW update
19:46:25 : Unpacking KDZ
19:46:55 : KDZ file extraced
19:47:07 : Files were extracted.
19:47:07 : LGMobileDL Load.
19:47:07 : Port = -1
19:47:07 : Connecting to phone
19:47:17 : Check Phone mode = 1
19:47:35 : Phone type check.......
19:47:35 : LGMobileDL.DLL Loading....
19:47:35 : _SetAuthMark Ok : wAuthMark(9698)
19:47:35 : _DetachDLL Call
19:47:35 : _DetachDLL Call End
19:47:35 : Param : Path = C:\Documents and Settings\All Users\Application Data\LGMOBILEAX\Phone\DZ_GT540AT-00-V10d-BAL-XXX-MAY-17-2010.dz
19:47:35 : Param : moduleDir =
19:47:35 : Param : waitTime = 0
19:47:35 : Param : UsbHighSpeed = 1
19:47:35 : Param : PhoneMode = 1
19:47:35 : Param : BinVersion = V10D_00
19:47:35 : Param : AuthMark = 9698
19:47:35 : Call fn_StartUpgrade
19:47:58 : 3GQCT : wParam = 4, lParam = 0
19:47:58 : Step Complete : DLOAD Change
19:48:38 : 3GQCT : wParam = 3, lParam = 0
19:48:38 : Step Complete : C Drive Erase
19:48:38 : 3GQCT : wParam = 205, lParam = 0
19:48:38 : 3GQCT : wParam = 205, lParam = 1
19:48:38 : 3GQCT : wParam = 205, lParam = 2
19:48:38 : 3GQCT : wParam = 205, lParam = 3
19:48:38 : 3GQCT : wParam = 205, lParam = 4
19:48:38 : 3GQCT : wParam = 205, lParam = 5
19:48:38 : 3GQCT : wParam = 205, lParam = 6
19:48:38 : 3GQCT : wParam = 205, lParam = 7
19:48:39 : 3GQCT : wParam = 205, lParam = 8
19:48:39 : 3GQCT : wParam = 205, lParam = 9
19:48:39 : 3GQCT : wParam = 205, lParam = 10
19:48:39 : 3GQCT : wParam = 205, lParam = 11
19:48:39 : 3GQCT : wParam = 205, lParam = 12
19:48:39 : 3GQCT : wParam = 205, lParam = 13
19:48:39 : 3GQCT : wParam = 205, lParam = 14
19:48:39 : 3GQCT : wParam = 205, lParam = 15
19:48:39 : 3GQCT : wParam = 205, lParam = 16
19:48:40 : 3GQCT : wParam = 205, lParam = 17
19:48:40 : 3GQCT : wParam = 205, lParam = 18
19:48:40 : 3GQCT : wParam = 205, lParam = 19
19:48:40 : 3GQCT : wParam = 205, lParam = 20
19:48:40 : 3GQCT : wParam = 205, lParam = 21
19:48:40 : 3GQCT : wParam = 205, lParam = 22
19:48:40 : 3GQCT : wParam = 205, lParam = 23
19:48:40 : 3GQCT : wParam = 205, lParam = 24
19:48:40 : 3GQCT : wParam = 205, lParam = 25
19:48:41 : 3GQCT : wParam = 205, lParam = 26
19:48:41 : 3GQCT : wParam = 205, lParam = 27
19:48:41 : 3GQCT : wParam = 205, lParam = 28
19:48:41 : 3GQCT : wParam = 205, lParam = 29
19:48:41 : 3GQCT : wParam = 205, lParam = 30
19:48:41 : 3GQCT : wParam = 205, lParam = 31
19:48:41 : 3GQCT : wParam = 205, lParam = 32
19:48:41 : 3GQCT : wParam = 205, lParam = 33
19:48:41 : 3GQCT : wParam = 205, lParam = 34
19:48:42 : 3GQCT : wParam = 205, lParam = 35
19:48:42 : 3GQCT : wParam = 205, lParam = 36
19:48:42 : 3GQCT : wParam = 205, lParam = 37
19:48:42 : 3GQCT : wParam = 205, lParam = 38
19:48:42 : 3GQCT : wParam = 205, lParam = 39
19:48:42 : 3GQCT : wParam = 205, lParam = 40
19:48:42 : 3GQCT : wParam = 205, lParam = 41
19:48:42 : 3GQCT : wParam = 205, lParam = 42
19:48:42 : 3GQCT : wParam = 205, lParam = 43
19:48:42 : 3GQCT : wParam = 205, lParam = 44
19:48:43 : 3GQCT : wParam = 205, lParam = 45
19:48:43 : 3GQCT : wParam = 205, lParam = 46
19:48:43 : 3GQCT : wParam = 205, lParam = 47
19:48:43 : 3GQCT : wParam = 205, lParam = 48
19:48:43 : 3GQCT : wParam = 205, lParam = 49
19:48:43 : 3GQCT : wParam = 205, lParam = 50
19:48:43 : 3GQCT : wParam = 205, lParam = 51
19:48:43 : 3GQCT : wParam = 205, lParam = 52
19:48:43 : 3GQCT : wParam = 205, lParam = 53
19:48:44 : 3GQCT : wParam = 205, lParam = 54
19:48:44 : 3GQCT : wParam = 205, lParam = 55
19:48:44 : 3GQCT : wParam = 205, lParam = 56
19:48:44 : 3GQCT : wParam = 205, lParam = 57
19:48:44 : 3GQCT : wParam = 205, lParam = 58
19:48:44 : 3GQCT : wParam = 205, lParam = 59
19:48:44 : 3GQCT : wParam = 205, lParam = 60
19:48:44 : 3GQCT : wParam = 205, lParam = 61
19:48:44 : 3GQCT : wParam = 205, lParam = 62
19:48:45 : 3GQCT : wParam = 205, lParam = 63
19:48:45 : 3GQCT : wParam = 205, lParam = 64
19:48:45 : 3GQCT : wParam = 205, lParam = 65
19:48:45 : 3GQCT : wParam = 205, lParam = 66
19:48:45 : 3GQCT : wParam = 205, lParam = 67
19:48:45 : 3GQCT : wParam = 205, lParam = 68
19:48:45 : 3GQCT : wParam = 205, lParam = 69
19:48:45 : 3GQCT : wParam = 205, lParam = 70
19:48:45 : 3GQCT : wParam = 205, lParam = 71
19:48:45 : 3GQCT : wParam = 205, lParam = 72
19:48:46 : 3GQCT : wParam = 205, lParam = 73
19:48:46 : 3GQCT : wParam = 205, lParam = 74
19:48:46 : 3GQCT : wParam = 205, lParam = 75
19:48:46 : 3GQCT : wParam = 205, lParam = 76
19:48:46 : 3GQCT : wParam = 205, lParam = 77
19:48:46 : 3GQCT : wParam = 205, lParam = 78
19:48:46 : 3GQCT : wParam = 205, lParam = 79
19:48:46 : 3GQCT : wParam = 205, lParam = 80
19:48:46 : 3GQCT : wParam = 205, lParam = 81
19:48:47 : 3GQCT : wParam = 205, lParam = 82
19:48:47 : 3GQCT : wParam = 205, lParam = 83
19:48:47 : 3GQCT : wParam = 205, lParam = 84
19:48:47 : 3GQCT : wParam = 205, lParam = 85
19:48:47 : 3GQCT : wParam = 205, lParam = 86
19:48:47 : 3GQCT : wParam = 205, lParam = 87
19:48:47 : 3GQCT : wParam = 205, lParam = 88
19:48:47 : 3GQCT : wParam = 205, lParam = 89
19:48:47 : 3GQCT : wParam = 205, lParam = 90
19:48:48 : 3GQCT : wParam = 205, lParam = 91
19:48:48 : 3GQCT : wParam = 205, lParam = 92
19:48:48 : 3GQCT : wParam = 205, lParam = 93
19:48:48 : 3GQCT : wParam = 205, lParam = 94
19:48:48 : 3GQCT : wParam = 205, lParam = 95
19:48:48 : 3GQCT : wParam = 205, lParam = 96
19:48:48 : 3GQCT : wParam = 205, lParam = 97
19:48:48 : 3GQCT : wParam = 205, lParam = 98
19:48:48 : 3GQCT : wParam = 205, lParam = 99
19:48:48 : 3GQCT : wParam = 205, lParam = 100
19:48:48 : 3GQCT : wParam = 205, lParam = 0
19:48:49 : 3GQCT : wParam = 205, lParam = 1
19:48:49 : 3GQCT : wParam = 205, lParam = 2
19:48:49 : 3GQCT : wParam = 205, lParam = 3
19:48:49 : 3GQCT : wParam = 205, lParam = 4
19:48:49 : 3GQCT : wParam = 205, lParam = 5
19:48:49 : 3GQCT : wParam = 205, lParam = 6
19:48:49 : 3GQCT : wParam = 205, lParam = 7
19:48:49 : 3GQCT : wParam = 205, lParam = 8
19:48:49 : 3GQCT : wParam = 205, lParam = 0
19:48:49 : 3GQCT : wParam = 205, lParam = 1
19:48:49 : 3GQCT : wParam = 205, lParam = 2
19:48:49 : 3GQCT : wParam = 205, lParam = 0
19:48:49 : 3GQCT : wParam = 205, lParam = 1
19:48:49 : 3GQCT : wParam = 205, lParam = 2
19:48:49 : 3GQCT : wParam = 205, lParam = 3
19:48:49 : 3GQCT : wParam = 205, lParam = 4
19:48:49 : 3GQCT : wParam = 205, lParam = 5
19:48:49 : 3GQCT : wParam = 205, lParam = 6
19:48:49 : 3GQCT : wParam = 205, lParam = 7
19:48:49 : 3GQCT : wParam = 205, lParam = 8
19:48:50 : 3GQCT : wParam = 205, lParam = 9
19:48:50 : 3GQCT : wParam = 205, lParam = 10
19:48:50 : 3GQCT : wParam = 205, lParam = 11
19:48:50 : 3GQCT : wParam = 205, lParam = 12
19:48:50 : 3GQCT : wParam = 205, lParam = 13
19:48:50 : 3GQCT : wParam = 205, lParam = 14
19:48:50 : 3GQCT : wParam = 205, lParam = 15
19:48:50 : 3GQCT : wParam = 205, lParam = 16
19:48:50 : 3GQCT : wParam = 205, lParam = 17
19:48:50 : 3GQCT : wParam = 205, lParam = 18
19:48:50 : 3GQCT : wParam = 205, lParam = 19
19:48:50 : 3GQCT : wParam = 205, lParam = 20
19:48:50 : 3GQCT : wParam = 205, lParam = 21
19:48:50 : 3GQCT : wParam = 205, lParam = 22
19:48:50 : 3GQCT : wParam = 205, lParam = 23
19:48:50 : 3GQCT : wParam = 205, lParam = 24
19:48:50 : 3GQCT : wParam = 205, lParam = 25
19:48:50 : 3GQCT : wParam = 205, lParam = 26
19:48:50 : 3GQCT : wParam = 205, lParam = 27
19:48:50 : 3GQCT : wParam = 205, lParam = 28
19:48:50 : 3GQCT : wParam = 205, lParam = 29
19:48:50 : 3GQCT : wParam = 205, lParam = 30
19:48:50 : 3GQCT : wParam = 205, lParam = 31
19:48:50 : 3GQCT : wParam = 205, lParam = 32
19:48:50 : 3GQCT : wParam = 205, lParam = 33
19:48:50 : 3GQCT : wParam = 205, lParam = 34
19:48:50 : 3GQCT : wParam = 205, lParam = 35
19:48:50 : 3GQCT : wParam = 205, lParam = 36
19:48:50 : 3GQCT : wParam = 205, lParam = 37
19:48:50 : 3GQCT : wParam = 205, lParam = 38
19:48:50 : 3GQCT : wParam = 205, lParam = 39
19:48:50 : 3GQCT : wParam = 205, lParam = 40
19:48:50 : 3GQCT : wParam = 205, lParam = 41
19:48:50 : 3GQCT : wParam = 205, lParam = 42
19:48:50 : 3GQCT : wParam = 205, lParam = 43
19:48:51 : 3GQCT : wParam = 205, lParam = 44
19:48:51 : 3GQCT : wParam = 205, lParam = 45
19:48:51 : 3GQCT : wParam = 205, lParam = 46
19:48:51 : 3GQCT : wParam = 205, lParam = 47
19:48:51 : 3GQCT : wParam = 205, lParam = 48
19:48:51 : 3GQCT : wParam = 5, lParam = 0
19:48:51 : Step Complete : Main Binary D/L to SDRAM
19:48:52 : 3GQCT : wParam = 205, lParam = 0
19:48:53 : 3GQCT : wParam = 205, lParam = 1
19:48:54 : 3GQCT : wParam = 205, lParam = 2
19:48:55 : 3GQCT : wParam = 205, lParam = 3
19:48:56 : 3GQCT : wParam = 205, lParam = 4
19:48:58 : 3GQCT : wParam = 205, lParam = 5
19:48:59 : 3GQCT : wParam = 205, lParam = 6
19:49:00 : 3GQCT : wParam = 205, lParam = 7
19:49:01 : 3GQCT : wParam = 205, lParam = 8
19:49:02 : 3GQCT : wParam = 205, lParam = 9
19:49:03 : 3GQCT : wParam = 205, lParam = 10
19:49:04 : 3GQCT : wParam = 205, lParam = 11
19:49:05 : 3GQCT : wParam = 205, lParam = 12
19:49:06 : 3GQCT : wParam = 205, lParam = 13
19:49:07 : 3GQCT : wParam = 205, lParam = 14
19:49:08 : 3GQCT : wParam = 205, lParam = 15
19:49:10 : 3GQCT : wParam = 205, lParam = 16
19:49:11 : 3GQCT : wParam = 205, lParam = 17
19:49:12 : 3GQCT : wParam = 205, lParam = 18
19:49:13 : 3GQCT : wParam = 205, lParam = 19
19:49:14 : 3GQCT : wParam = 205, lParam = 20
19:49:15 : 3GQCT : wParam = 205, lParam = 21
19:49:16 : 3GQCT : wParam = 205, lParam = 22
19:49:17 : 3GQCT : wParam = 205, lParam = 23
19:49:18 : 3GQCT : wParam = 205, lParam = 24
19:49:19 : 3GQCT : wParam = 205, lParam = 25
19:49:21 : 3GQCT : wParam = 205, lParam = 26
19:49:22 : 3GQCT : wParam = 205, lParam = 27
19:49:23 : 3GQCT : wParam = 205, lParam = 28
19:49:24 : 3GQCT : wParam = 205, lParam = 29
19:49:25 : 3GQCT : wParam = 205, lParam = 30
19:49:26 : 3GQCT : wParam = 205, lParam = 31
19:49:27 : 3GQCT : wParam = 205, lParam = 32
19:49:28 : 3GQCT : wParam = 205, lParam = 33
19:49:29 : 3GQCT : wParam = 205, lParam = 34
19:49:30 : 3GQCT : wParam = 205, lParam = 35
19:49:31 : 3GQCT : wParam = 205, lParam = 36
19:49:33 : 3GQCT : wParam = 205, lParam = 37
19:49:34 : 3GQCT : wParam = 205, lParam = 38
19:49:35 : 3GQCT : wParam = 205, lParam = 39
19:49:36 : 3GQCT : wParam = 205, lParam = 40
19:49:37 : 3GQCT : wParam = 205, lParam = 41
19:49:38 : 3GQCT : wParam = 205, lParam = 42
19:49:39 : 3GQCT : wParam = 205, lParam = 43
19:49:40 : 3GQCT : wParam = 205, lParam = 44
19:49:41 : 3GQCT : wParam = 205, lParam = 45
19:49:42 : 3GQCT : wParam = 205, lParam = 46
19:49:44 : 3GQCT : wParam = 205, lParam = 47
19:49:45 : 3GQCT : wParam = 205, lParam = 48
19:49:46 : 3GQCT : wParam = 205, lParam = 49
19:49:47 : 3GQCT : wParam = 205, lParam = 50
19:49:48 : 3GQCT : wParam = 205, lParam = 51
19:49:49 : 3GQCT : wParam = 205, lParam = 52
19:49:50 : 3GQCT : wParam = 205, lParam = 53
19:49:51 : 3GQCT : wParam = 205, lParam = 54
19:49:52 : 3GQCT : wParam = 205, lParam = 55
19:49:53 : 3GQCT : wParam = 205, lParam = 56
19:49:55 : 3GQCT : wParam = 205, lParam = 57
19:49:56 : 3GQCT : wParam = 205, lParam = 58
19:49:57 : 3GQCT : wParam = 205, lParam = 59
19:49:58 : 3GQCT : wParam = 205, lParam = 60
19:49:59 : 3GQCT : wParam = 205, lParam = 61
19:50:00 : 3GQCT : wParam = 205, lParam = 62
19:50:01 : 3GQCT : wParam = 205, lParam = 63
19:50:02 : 3GQCT : wParam = 205, lParam = 64
19:50:04 : 3GQCT : wParam = 205, lParam = 65
19:50:05 : 3GQCT : wParam = 205, lParam = 66
19:50:06 : 3GQCT : wParam = 205, lParam = 67
19:50:07 : 3GQCT : wParam = 205, lParam = 68
19:50:08 : 3GQCT : wParam = 205, lParam = 69
19:50:09 : 3GQCT : wParam = 205, lParam = 70
19:50:10 : 3GQCT : wParam = 205, lParam = 71
19:50:11 : 3GQCT : wParam = 205, lParam = 72
19:50:12 : 3GQCT : wParam = 205, lParam = 73
19:50:14 : 3GQCT : wParam = 205, lParam = 74
19:50:15 : 3GQCT : wParam = 205, lParam = 75
19:50:16 : 3GQCT : wParam = 205, lParam = 76
19:50:17 : 3GQCT : wParam = 205, lParam = 77
19:50:18 : 3GQCT : wParam = 205, lParam = 78
19:50:19 : 3GQCT : wParam = 205, lParam = 79
19:50:20 : 3GQCT : wParam = 205, lParam = 80
19:50:21 : 3GQCT : wParam = 205, lParam = 81
19:50:22 : 3GQCT : wParam = 205, lParam = 82
19:50:24 : 3GQCT : wParam = 205, lParam = 83
19:50:25 : 3GQCT : wParam = 205, lParam = 84
19:50:26 : 3GQCT : wParam = 205, lParam = 85
19:50:27 : 3GQCT : wParam = 205, lParam = 86
19:50:28 : 3GQCT : wParam = 205, lParam = 87
19:50:29 : 3GQCT : wParam = 205, lParam = 88
19:50:30 : 3GQCT : wParam = 205, lParam = 89
19:50:31 : 3GQCT : wParam = 205, lParam = 90
19:50:33 : 3GQCT : wParam = 205, lParam = 91
19:50:34 : 3GQCT : wParam = 205, lParam = 92
19:50:35 : 3GQCT : wParam = 205, lParam = 93
19:50:36 : 3GQCT : wParam = 205, lParam = 94
19:50:37 : 3GQCT : wParam = 205, lParam = 95
19:50:38 : 3GQCT : wParam = 205, lParam = 96
19:50:39 : 3GQCT : wParam = 205, lParam = 97
19:50:40 : 3GQCT : wParam = 205, lParam = 98
19:50:41 : 3GQCT : wParam = 205, lParam = 99
19:50:43 : 3GQCT : wParam = 205, lParam = 100
19:50:43 : 3GQCT : wParam = 8, lParam = 0
19:50:43 : Step Complete : Phone Reset(DLL Cmd)
19:51:23 : 3GQCT : wParam = 12, lParam = 0
19:51:25 : Step Complete : UPDATE FINISHED!
19:51:25 : ResetStartInfo() Call
19:51:25 : _DetachDLL Call
19:51:25 : _DetachDLL Call End
19:51:25 : FreeLibrary Call
19:51:25 : FreeLibrary Call End
19:51:25 : ResetStartInfo() Call End
19:51:25 : ===FINISHED===

If you have any problems I would be glad to help