Nevermind all! My cancellation went through.
MotorsportReg, I think, allows you to sign up a different driver, but they just have to buy a BMW CCA membership if they don’t have one. So, I think you can attempt to sell the spot (I wasn’t trying to do it for a profit, just trying to not lose $50), but it’s ultimately up to the coordinators if they’ll let that fly.
Or you can cancel and lose $10, and get the rest back in credits.