You need a celestial schism mold from the boutique. and 200 Mirage Celestones.
Bring them to the Conversion Executor in Thousand Streams, and take the available quest.
When you have the Celestial Schism, press the Conversion Service at same NPC.
Yes, it will reset all of your demon skills, as well as any Primal skills. It will also wipe your skillbars, so save your setup so you can redo it if you don't have it memorized.
