Just think about updating Microsoft Office or another software! It is basically that you replace an old one with a new one which is like copying a file over another. Consider this as a delete and recreate! Microsoft Office allows you to save your docs but these are your personal files. In gaming, it comes true what Antikore is saying. You can think of running your own old files on new features but those may become really incompatible with your updates. Also, think about how much it even messes up your work if you run your game on a different version of PB and you want to run save files on updated courses?!
And think about how difficult it would be to program it if save files are probably just files from a game state?
I have to say that don't expect this suggestion to be accepted ever.
Deleting save games if updating game is not a bug nor glitch because you will be warned about it and it is normal. Saving gaming progress have been meant to be possible mostly for complete game and a lot of people will not publish their games before ready but we are doing it just to share our content and see what people have to say.