Saves before 0.31 are not compatible with versions 0.31 and above, and saves 0.31 and above will be compatible with others. We just recently overhauled the system, so that's how it is now.
That is, if you make a save in 0.31, you can play 0.32 without problems with the same save.
We talk more about that here:
https://www.patreon.com/posts/development-0-31-74304390