Sometimes characters change classes as part of the story (tho it's mostly just the name, like Jeanne starting as a Paladin with no spells but revealing she is actually a Dark Paladin and finally being able to cast spells). If Mao or Lianna ever do it in the future, it's probably just gonna be an "upgrade" to their current one.
The first 8 classes in the game date are the default ones that come ready in RPGM. I only use them as templates for my custom ones xD
That said, you can expect to see more playable characters in the post game. I have 2 more planned, but no promises rn ;)
(Btw, I'm just finishing up translating the next update. It should come out this weekend!)