I've gotten upgrades twice as Silver (from outside-facing rooms to atriums....which are almost always more preferred), but only because it was early in the check-in window and they were looking to get the line moving, it seemed. Once, it started with an offer for an upgrade at $40/night (declined) and then became "since you are Elite....". Seemed to be an informal policy, of sorts, but I took it and was happy, knowing I would have gotten nothing else either time.
This would be during high-volume check-in time for convention starts, versus leisure periods.