Hi all,
Good advice to back up your project before switching engine versions.
To clarify a little bit, say you start developing/experimenting with a 5.2 preview version.
You will be able to upgrade your 5.2 preview version install to ‘5.2.0’.
Certain functions may break (Engine developers can change more things during preview versions than with hotfix versions) but I don’t think you’d ever be in a position where you’re ‘starting from scratch’
Unless there is a new feature in 5.2 that is foundational to your project. (PCG? 
) I’d say to start developing in 5.1, but open a copy of your project in 5.2 preview to check for bugs introduced by the new version. (If you find an engine regression/bug there is the possibility of reporting it during the preview period and having it fixed by release- instead of waiting until 5.3)
Good luck!