Separate RPG class experience

So I'm trying to make an RPG game where after you gain enough exp on a job class you will have "Mastered" the job class which will then allow you to advance or change your class to something else. For example you start the game as a Warrior, gain exp from defeating monsters, and after you max out the exp bar you can then change to Mage, or advance into a Knight. Also if a player maxes out 2 classes, like mage and warrior, they can unlock a new Spellsword class to become as well.

My issue is that I want to set up a code that stores the exp from one class in case the player decides to change classes before maxing out the exp. Let's say the max exp is 10. Player obtains 5 exp as a Warrior but then decides to change to a Rogue and then gains another 3 exp, but then realizes that Warrior was a better path so then the player goes back to Warrior. The game will know that Warrior for this player had 5 exp before switching and will return that 5 exp while also retaining the 3 exp for Rogue if player ever decides to switch back.

I'm still fairly new at Unity so any help would be appreciated. Also I don't know if I should be doing this or not but I have a ton of classes, (167 total!) so I set up an array to support them all. Should I be using arrays?