With a little help of you guys, I have realized simpliest way to do that (works in YAN's Battle System)
1. Make sure you have sprites for your normal character and sprites for transformation you want to get him in
2. Create a new state and name it as your transformation
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5. In Transform skill, add state and common event meant for it (make sure that you use Event to Forget transform skill, so multiple transformations/extra buffing can't be done)
6. In Changeback skill, remove state and and add event meant for it (make sure that you remove Changeback skill in that event, and state aswell + Add back your transform skill)
If it's confusing, I'll try to make a video
This is a beginner tutorial and advanced users would probably know how to do it!