They will gain experience, but it resets when you change armors.

(For some reason, I read "battleset" as "charset".. when you meant "Battle Charset".)

This is one of the oddest things with RM2k3 - there's actually no way to swap Battle Charsets outside of a class change. If you want to preserve your EXP, do the following:

<> Variable Oper: Set [0020:CubeEXP] Cube Experience
<> Change Class: Cube SuperArmor, Change to Level 1, No Skill Change, Stats Level 1 New Class
<> Change Experience: Cube's Experience /[0020:CubeEXP] Add
<>

----

That will keep the same EXP from before the class change. I'm still not sure why Enterbrain forgot the "little" detail of Battle Charsets within events, but at least there's a way around it.