Better start new character - other way:
1. You must do 20 lvl
2. Take Ressurection Potion Quest (something like that)
3. Collect big amount of cursed hearts (10x skills and mastery lvl that you want to dlvl)
4. Get a LOT of money to realocate sps
5. Voila, you get ONLY 80% of your sps... so if you used ~2k of sp you get only 1,6k... quite a lot sp you lost
6. It's not worth to use that to change class - i know if you will be on 60+ lvl - yep you can try that but you will be very much weaker than others on that level unless you farm that losted sp's
