[T-Mobile LG G2x] Core Memory:
There's virtually no risk, I've done it a few times without any problems.

Set up the phone as follows:

1) Settings -> Applications -> Development -> enable USB debugging

2) Access the phone's hidden menu using the phone keypad, enter: 1809#*999#

In Port Setting enable CP USB (do not enable Retain ...) then press OK

3) Plug your phone into the USB port and wait a few seconds until the USB drivers install.

4) In the terminal port setting, get the LGE Mobile USB Modem COM Port number by accessing the USB driver through Control Panel -> Device Manager -> LGE Mobile USB Modem -> Advanced -> Advanced Port Settings -> COM Port Number

Write down the port number.

5) Download RSterm and use it, that's what I used:

http://www.roundsolutions.com/techdocs/#tools

Install RSterm and run it.

6) In RSterm, where it states "Serial Port setup", through the scroll box, select the LGE Mobile USB Modem Port Number you wrote down and click Open.

Put a check mark in the box next to "Local echo".

In the terminal RSterm terminal window, type: AT

The terminal should respond with OK

If it doesn't it is probably because the selected port is incorrect. Recheck that the Port Setting accessed through the phone's hidden menu is CP USB (step 2) , if it isn't enable it and redo step 6.

You may click on some of the Modem command buttons on the left side which send AT commands to the modem but only the following ones which do not change any settings:

AT, AT+CPIN?, AT+CREG?, AT+CGATT?, AT+CGMR

The modem should respond with either OK or information about the modem.

8) Now you may enter the AT command for changing the phone's modem band setting.

(It is important to use 0 (zero) in the AT control string so that the change is temporary and the setting will revert back to normal when you reboot your phone.)

Enter this in the terminal to enable all the bands (46) that are available in the G2X which the firmware allows:

AT$QCBANDPREF= 0, "46"

Here are the WCDMA band number codes and their descriptions for each one.

"27.WCDMA_I_IMT_2000"

"28.WCDMA_II_PCS_1900"

"29.WCDMA_III_1700 "

"30.WCDMA_IV_1700"

"31.WCDMA_V_850"

"32.WCDMA_VI_800 "

"33.WCDMA_VII_2600"

"34.WCDMA_VIII_900"

"35.WCDMA_IX_1700"

"46.Any"

9) In the phone's hidden menu, click on: Engineering Mode and press Next until page 3 appears then scroll down to ITEM_NM : 3 where SW EnabledBand is located where you will see 0x...

(You will also see that the terminal window displays that page in real time)

The SW EnabledBand indicates the band selection code which should be 0x0000000102400000 which is a change from the default 0x00000001020000000

10) Press the return led button at the bottom of the phone to get back to Band Selection in the Modem Setting function of the hidden menu then press Band Selection and set it to Automatic.

11) Press the Home led button at the bottom of the phone then access Settings -> Wireless and Networks -> Mobile network settings -> Network operators -> Search networks and see what it finds.

12) You can reset the phone back to its default band settings by rebooting the phone.

13) If you had any other settings which were changed, such as changing from Automatic band selection back to whatever you had it set to before, you will have to reset them manually.