Can you save the prize tokens? or do you have to exchange them for donuts?

Ok, so I screwed up and ended up spending a bunch of donuts for the 5 pack of prize tokens - Only to realize that I could have waited and gotten them for free while completing the tasks!!!! UGH! I already spent 4/5 tokens I bought on the things I didn't have.
Question is, I have one left and will be getting more from the event - do I have to exchange them for donuts? Or can I save the tokens for the next event and redeem them for something?