Hi,
The definitive answer comes from an Engine Contributor
in another post.
UE 5.1 in GitHub ue5-main and backwards compatibility with 5.0 -
Unreal only implements forward migrations, so if you use the dev branch, you will not necessarily be able to open the project in a release branch for even the same version.
You should keep a backup of your original Project in a new separate directory.
When the question about UE5 conversion comes up always select “make a copy”. If convert in place you will possibly corrupt many assets.
The backward compatibility of any prior/older version is not supported, except by making a backup copy of the project and this applied right back to version UE 4.14