I just leased a Pruis and have been wondering the same thing. Should I change the agreement to a purchase? I don't think it matters since I can purchase at lease end. Why commit now? You have the car for 3 years anyways. Honda lets you buy it at anytime and I suspect Toyota will also. They set the price and don't have to dispose of the car so why wouldn't they? I changed a Honda lease to a purchase about 2 years in. No problem. Just sign the papers and decide while you're making those cheap lease payments. You may want a new Prius in 3 years not your "old" one. Who really knows? That's what I plan on doing.
Thanks for all the great responses. I went in and signed the lease and will think about what I want to do down the road.