also, with diamond suite upgrades, it's always a good idea to secure them as far in advance as possible. What I typically do is book a cancellable rate (if you're a costco member, try the costco rate as it's always as good or better than the advance purchase rate but can be cancelled -- if it's available), apply the diamond cert (assuming, again, that it's available) and then monitor the rates. Once you are sure you will be making the trip, if you find an advanced purchase rate that's lower than the cancellable rate, call or Tweet @hyattconcierge and ask them to change your reservation to a pre-paid rate to save some $$. your upgrade will not be affected.
good luck!