Right now, it's technically possible to upgrade any image flavor to any other flavor. In the time when many flavors were supposed to be upgraded to the generic ISO, it would be difficult to determine "matching" flavors.

However, the problem of flavor compatibility has existed for a long time: certain upgrades could break the system badly. For example, upgrading EdgeCore or Dell VEP images to a generic ISO could throw off interface numbering. Upgrading Azure instances to an image without WAAgent would make the VM lose many platform service like backups and proper stats reporting.

Recently a community member demonstrated that non-matching upgrades can even lead to security problems (https://forum.vyos.io/t/really-dangerous-1-4-1-bug-default-password-came-back/15785/2).

Now that we have a flavor field in the version data file and we have a way to produce targeted flavors for all platforms easily, it's time to only allow matching upgrades.