Upgrading Qubes-Whonix 13 on Qubes R4 to Qubes-Whonix 14 has been briefly tested. The upgrade should run smooth. However, it needs a lot more testing.
Same should apply to Qubes-Whonix 13 on Qubes R3.2 to Qubes-Whonix 14 which was implemented earlier and probably not broken by the latest changes (bug fixes for Qubes R4 support).
Non-Qubes-Whonix 13 to Non-Qubes-Whonix 14 upgrades should be possible as well, but less tested at this point. Help welcome!
All the tickets there were implemented for Whonix 14 need to be tested.