Works just fine. I did exactly what you said you want to do, and I'm back to GB again, for now. Do this:
- Create a nandroid backup of 1.5
- Copy your SD Card to a PC (just in case)
- Boot into the bootstrapper and install the GB update.zip
- If you actually boot into GB (may take 10+ minutes), go through the motions until you can power off. If you bootloop, that's okay, skip this step.
- Boot into recovery (hold Power and Home until you see the exclamation screen)
- Press Volume UP and Volume DOWN at the same time, while you're on the exclamation screen
- Do a factory reset (use volume buttons to move, power button to OK)
- Reboot and have fun with GB!
- When you're ready to go back to Froyo, SBF the 340 full SBF file, boot up, activate, root, install bootstrap, wipe data, then restore your backup.
- Upgrade to Liberty 2.0 when it's ready!