I don’t know where to find the save files or why you want to find them.
But normally the new version just use the same place for the saves, so that it shows the old ones (and use them)
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.