I'm fairly sure the rewards carry over. I had a female witch who had the magic hands lifetime reward who I turned into a human when she was an elder because I wanted her to cane around, lol. I remember her having the magic hands reward just useless in the reward UI. However, do a quick test (save, have lover ask to give up powers, do opportunity and check if he still has the reward - if it disappears just quit without saving) just to be sure.