well it's not supported.. and potentially it could break something..depending on if you used a new feature that is not in an older version.. but 99% of the time it should be fine..
i.e. if your project uses Local Storage and you change it from version 20300 to version 18700, you will have a problem since Local Storage was not in C2 at that time. I don't know if there's a way to take out the new events, but I wouldn't want to mess with it.
I mostly change the caproj for downloading other people's posted capx... everyone seems in a hurry to update, I'm always way behind..