[T-Mobile myTouch 4G Slide] tbalden:
THIS IS FOR S-On Unlocked. -- WE STILL NEED THE PD...IMG.ZIP, Blue6ix will provide it here later, then ill modify this guide.

OKAY, so what I write here is all theory, but i've created a tool that really modifies the MISC partition to fake a lower version number for your device, so that older system update zip can be tried to be flashed from Bootloader on your device, downgrading to a system which can be S-Offed by the Revolutionary method.

Noone tried this yet, I only can assure that this tool worked on my MISC image, set the version number in the correct position of the MISC partition.

BUT IN ANY CASE, IF YOU ARE BRAVE ENOUGH TO TRY THIS METHOD, BE PREPARED THAT YOU MIGHT BRICK YOUR PHONE. NOONE ELSE TRIED THIS ON AN S-ON DEVICE YET.

But methods similar to this one were succesful with other HTC devices like Desire Z/DHD for downgrading...

Okay, so ...

Here's the misc_version tool modified for mt4gs. It will write directly into your /dev/block/mmcblk0p31 partition, but before it is going to create a backup of it on your sdcard.

http://www.4shared.com/zip/ZFKef2Fq/misc_version_02_mt4gs.html

First make sure that you have an SDCard with a PD...IMG.zip (soon to have the link from Blue6ix) copied onto it to its root, like this:

adb push PD...IMG.ZIP /sdcard

After that IMG.zip is in place, download this zip named misc_version_mt4gs.zip, unzip it, and push misc_version binary to you device

adb push misc_version /system/bin
adb shell
~ su
# chmod 775 /system/bin/misc_version

Then to use it actually:

adb shell

~ su
# misc_version -s 1.00.000.0
--set_version set. VERSION will be changed to: 1.00.000.0
Patching and backing up partition 31...
#

This is what you should see if everything went right.

After that you'll need the PD....IMG.zip Blue6ix might soon link here. (Read the first part where we pushed that Zip to sdcard)

So if thats in place you will need to power off, and do the boot into bootloader with Power + VolDown. Then Bootloader will see that the Zip is on the sdcard root directory, and hopefully it will patch successfully your device back to the S-Offable version. (After that take the sdcard off, possibly delete the update zip before you reinsert it to your phone.)

If the update was succesfull, start your phone, see if boots (lets hope so :D)) if everything is right (check About Software after booting/setting up) proceed to Revolutionary method and S-Off your device.

AGAIN I can't assure you that after running misc_version, it will boot after that modification on an S-On Unlocked device, but we all hope for that.

AGAIN, we need the pd...img.zip yet, and you must be S-On unlocked/with su installed.

ALSO IT WILL CREATE A BACKUP OF YOUR partition 31 on your sdcard, which you can 'dd' back if something went wrong. (If something went wrong but your phone is still bootable to recovery, like Clockworkmod, you can use adb shell to get to a shell and there you can 'dd' the backup to the partition, but lets hope it wont be needed)

AGAIN YOU CAN BRICK YOUR PHONE. THIS IS ALL THEORY, THAT WORKED ON HTC G2 (Desire Z) and Desire HD. Read the thread, ive linked those howtos for those other devices. (like this: http://wiki.cyanogenmod.com/wiki/HTC_Desire_HD:_Firmware_Downgrade_(Gingerbread) )