Should be Ok actually, updates should not penalise a vault, so it will be able to keep going (or should not at least). However a vault that did not update would probably be slower and after 2 updates would be removed form the network. Something for the update strategy though to take care of. It is though again another reason to not expect too many nodes age 255, even if we did reduce the age of all restarting vaults on update it would be network wide as nodes upgraded, so net net the end result would be a node not penalised for upgrade wrt to all other nodes.
Still some edge cases, but not a worry I think.