True, but it shouldn't happen after you've checked into the room. Or, if for some reason there's a good reason to do so, I would hope Marriott would call and explain the situation, not wait until you come to the front desk.

I think I would have raised a little hell on this one (and I rarely do that, maybe once every few years). In the best case, it's really poor customer service or policy implementation. What if the OP had been settled into his room, unpacked, and Marriott decided to give it away to someone willing to pay? Once you're checked in, expectations have been set.

If nothing else, change the rule to not upgrade until the person physically presents himself at the front desk. Better to upgrade late than to have it taken back.