In the game I'm making, characters can be upgraded by level and by tier. Each tier is the same as the last one but with higher base stats, critical chance, and so on. However I have a big problem. When a character's class is changed (which means when I upgrade that character's tier) the character goes back to level 1 which pretty much ends up being a step backwards temporarily until the player can level up (which would be a useless, progress-halting task). How would I do that a character keeps their EXP upon being changed class?