you can move between the release channels, but it implies a user-data reset (data loss) when the phone is locked and needs to be un/relocked. You need to use the same versions / or ahead in target to hop over because of rollback protection.
implications: do you do something that anyone would spend a non trivial amount of money to target you? if not, have apps and browser updated and if you can, weed unneeded apps.
the ETA on A13-A15 of this week for official:
(In general, I’m sceptical of the “2 more years of support for FP3” by murena as in: I don’t think this includes the firmware bits)