We do recommend to keep a backup copy of the previous version and check if you have no issues in the new version.
While doing this is you encounter any issue or new bug, you still have access to your previous version file.
oh yes, Ankus, i always keep a backup (or two, lol).
I just don't want to encounter not being able to open my sites if they are a couple of versions behind the next release.
No you will not be able to open Muse files made in the newer version with an older version.
If you have website updates to do when a new release of Muse comes out. Make all your updates, then download new Muse version, then when you reopen the Muse files again save as a new file.
I think the question is the other way round, Mike: It is possible to open .muse files of older versions in a new version, even if there are a couple versions between them. I manage to open documents of the first Muse release in the newest version.