Just an insight for anyone booking directly thru the Hyatt website.
I was booking a room for our upcoming Disneyland trip (I needed a larger room and couldn't use Priceline to get what we needed) and would occasionally check the website for specials. I noticed that the pricing/special offers for our week were changing so I started checking numerous times a day and found that the prices could change several times during a day. I ended up rebooking our room 3 times to get the additional savings. I ended up saving $200 for the week of our stay and upgrading to a larger suite.
The prices have since gone back up and my current rate (cancellable within 24 hours of arrival) is the same as the advance purchase rate.
Just thought this might be helpful.