A while ago we introduced a check for flavor match on upgrade that throws an error if the flavor name of the new image doesn't match the installed one.

The only way to bypass the check is to invoke the installer script with --force outside of the normal VyOS CLI. That's fine for unexpected situation but not very nice for situations when a flavor gets deprecated and supplanted by a flavor with a different name.

To make it possible to provide a migration path without asking users to invoke scripts directly, we could introduce a concept of "compatible flavors" and make the installer check if the new image flavor name is in the list in flavor.json.

If the name matches the installed image or any of the names listed as compatible, the upgrade can proceed. This way we can tell people to upgrade to a generic image, for example, when we deprecate a flavor—we just need to make sure to add it to the list in the flavor definition when we build the last release to support that flavor.