OK, just an update to close off this thread. Thanks for prior comments.
I deleted DVDfab v5.x using StornJumper's good documentation (see his sig for link), and cleaned the registry of 5.x entries (ccleaner). But one of the deleted 5.x entries must handle registration, because the next time I went to use 6.x, I was on a trial version despite it having been registered prior to all the deletions.
No sweat, just pasted in my key again. So, deleting 5.x in a dual 5 & 6 situation doesn't seem to break anything/too much, and is easily repairable in any case.