The save files currently produced are delicate arrangements that require a hermetically sealed environment to maintain their validity, I’m afraid.
We’re working on improving that, and did make some improvements in 0.4. However, that also had the side effect of making the game flat-out refuse to start if you load it with a 0.3 save folder.
So, yeah, keep playing 0.3 until that character dies, then start fresh with 0.4.