It is important to make a distinction between QuickTime and MoviePlayer. You can only ever install ONE version of QuickTime, but you can have as many copies of MoviePlayer/QuickTime Player on your disk as you wish. So if you want to keep full screen playback of QuickTime content, just keep MoviePlayer 2.5 around and install whatever version of QT you want. MoviePlayer uses whatever QuickTime extension is installed to do the decoding, but it isn't crippled like later versions.

Peace,

Drew