Currently, a Hyatt Diamond Suite Upgrade can be used towards reservations past the expiration date of the Upgrade, provided that no changes are made to the reservation after the expiration date. For example, this year’s DSUs expire on February 29, 2016, but if you apply them before that date, they can be used for reservations on March 1st, 2016, or any day after.
I have confirmed from Hyatt spokesperson that, starting with Diamond Suite Upgrades received on March 1, 2016, they can not be used on any reservations after the expiration. They will expire on February 28, 2017, and they cannot be used on reservations for March 1st, 2017, or any day following.
Since reservations can be made up to thirteen months in advance, this brings up the bizarre case where a March 2017 reservation can be upgraded to a suite using a 2015 DSU, but not with a 2016 DSU!
Probably the worst devaluation of benefits since I have become a Hyatt Gold Passport Diamond member.