I doubt there will be more program updates for v.6. Most probably it won't, even if some serious bug is found, but I might be wrong.
Regarding engine and definitions updates, they are the same for avast 5 / 6 / 7, so there is no problem with that.
Moreover, moving all current users from avast 6 to avast 7 will take some time (several weeks at least, if not more), so no need to rush.
Considering that avast 4.8 is still being used...
In any case, I'd rather see all the Windows 2k issues in relation to avast 7 resolved, including all the new supporting articles that are not explicitly saying that "Windows 2000 SP4 UR1" is supported (which should be included).