I play since r7.3 and I can only recall one time where an old save wasn’t usable in the new version. The save still existed but there was a bug, r20.1 ended exactly before meeting Fawn and if you continued a character from r20.1 in r21.1 that already crossed a certain point (I think the hard BDSM-session but I’m not sure) Fawn didn’t show up in the college.
Basically yes. At least on a windows desktop.
All I do is just unpacking the ZIP in the folder I use for LD+ and other adult games.
After that I can just delete the old folder and done.
As an Example the ZIP for the r27.1 has contains a folder named “Lust Doll Plus r27.1 (Win)” that contains all data, and (after unpacking) I just had to delete the folder “Lust Doll Plus r26.1 (Win)” so that it didn’t occupied storage.