Perhaps you have NEVER EVER noticed that there is a missing feature (bug?) in all of the RPG Maker editions regarding the class change and its corresponding skills.
Pick either XP or VX ACE or even MV and you will get the same result, your hero could change his or her class indeed... at a cost of not learning anything already listed in his or her new skillset!
NOTHING!
Here comes my scriptlet to let you solve this issue in a couple of creative ways!
Do Nothing! - Default (Still Able to Learn Skills in the Future)
Replace Old Skills with New Skills.
Add New Skills while Keeping the Old Skills.
Keep Some of the Old Skills while Learning the New Skills.
Free for use in any kind of game
Include my nickname in your game credits!
You aren't a blue squirrel desperate for a cup of coffee or a kitten that leaves you to caress it all night long, right?