In the latest update (EA3 at the time of this post) you can no longer convert your .brs files from pre-release within the game itself.
This is a bit of a shame because the .brs format was much simpler for tool makers like myself to work with. Anyways there is now a website where you can do the conversion to the new prefab format .brz. It doesn't upload the saves anywhere. It is just a WASM app that runs locally.
https://brs2brz.kmschr.com/
