For the record, you WILL be able to safely transfer your save files from 0.16 to 0.17 and 0.18, because those three are actually just one update that got split into three. But when it comes to save files from 0.15, use those on 0.16 at your own risk.
But you MUST make a copy of the save file folder before you delete the old version, because unlike with engines like Ren'py, which store the save files in a separate folder in your documents, RPGMaker keeps it's save files inside the actual game folder. So, deleting the game will also mean you lose the save folder.