Well, it seems like your problem is you will have a fighter (as an example class), and throughout the game the fighter may die, and you would like to have a new character, also a fighter, join to game to take Fighter_1's place.
Fighter_2 has a different appearance, exp curve, MaxHP, MaxSP etc parameters, and different starting equipment. Essentially, Fighter_2 is a new actor, but shares the same class as Fighter_1.
So my Theif_1 is Dorothy, she starts off the game with 0 exp
She gets in fights
And gains exp
As you can see, she has exp from battle
and for whatever reason she dies and is removed from the party.
Now a new thief emerges; the brave, strong, and incredibly handsome NR, and because he is a theif he got all the exp that Dorothy (also a thief) got from battle.
By having everyone in the class sharing exp, I thought that that would fulfil your original requirement:
So If the Characters and the classes level up the game would no longer be impossible.
Let me know if I'm completely wrong, I'm happy to make another script.