This is rarely brought up by people but worth pointing out: in legacy some myth units change visuals with upgrades but many do not.
Basically all upgrades that lead to a name change should also change unit visuals.
The list is not exhaustive:
Minotaur
Manticore
Nemean Lion
Almost all Egyptian ones (some have unused upgrade models; and don’t forget Sphinx with two upgrades but zero visual change)
Automaton
Hope this will be fixed in Retold.