Even if you prepaid with an adv. purchase rate, it is still worth a try re-checking rates. I've successfully requested switching to a lower rate (via Dia phone line) after previously booking an adv. purchase rate a couple times. If the savings are big enough, then it's worth a few minute phone call to at least try. Worse case is, they say no, and you end up staying anyway at the rate you had planned on paying.

I suspect the CS agents have a lot of latitude in resolving customer requests (a good thing, too!), so YMMV depending on agent. I suppose if what you're asking isn't too outrageous (i.e. you just rebook the same rm type/pkg or another prepaid, just at a lower rate), and you're a status member with lots of history, it's probably well worth it to Hyatt to let you save a few bucks. And, rebooking isn't the same as cancelling outright, anyway -- you're still going to end up staying.