Depends on what you mean by upgrade certificates. There are upgrade award certs that can be ordered for some hotels. But for the most part, the upgrade certificates would be for an upgraded view, not a suite upgrade. Back in the bygone era of the CY Club, one could earn suite upgrades for use at CYs. But those have been gone for quite a few years now.

Check out Room Upgrade Information at Marriott's website for more info.