Shifting major versions is an almost painless experience. So much so, that we forget all the places were we've specified something happen via MCXX.exe
I've already solved this problem!
Check out MCUtilities:
http://yabb.jriver.com/interact/index.php?topic=81514.0Included is MCcl.exe. This is simply a wrapper for MCXX.exe. You can run it once with the command line:
MCcl.exe --updateand it'll update itself to now use the version of MC currently registered with COM (which is typically the most recent version launched). Otherwise, calling MCcl.exe will automatically forward any command line parameters to the currently "associated" version of MCXX.exe.
So, all of my scripts call MCcl.exe instead of MC20.exe directly. When MC21.exe comes out, and I'm ready to "switch to it" permanently, I'll just run MC21 once (and approve the UAC prompt) to get it registered with COM, and then:
MCcl.exe --updateOne time. Done. All the scripts are automatically fixed. It works very, very well.