All you have to do is create a state and add any buffs you want under traits. There is even the option to add/seal skills and skill types there that would be undone when the state is removed. Create a skill that applies said state in whatever fashion you desire as well as switching character graphics. The graphics can be changed with the change actor images command using a common event (p.3 under system settings). All functions for removing the state can be controlled from the state screen, including how it is removed, whether by turn, damage, battle end, etc. After that all you have to do is make an event in every battle that changes that character graphics back to what they were before you transformed using a conditional branch.