I suspect the OP has one or more single night e-certs probably from either their CC or from the Megabonus.

And while I would agree that it would be great if Marriott would allow us to "upgrade" those CC/Megabonus e-certs, I'm very skeptical about it ever happening. Doing that would change the whole dynamics of those e-certs. They would become much more useable. It is very possible that many of them would then be used at the higher end properties (where awards tend to be booked more often already.) And I wonder how many of these e-certs end up never being used at all? If the Cat level can be increased (even with a cost), then I would guess there would be less e-certs going unused.

The end result is their costs would increase (yes, they might be able to offset that some by "charging" for the upgrade), but with that increased cost will they really see more business driven to either the Credit Cards or more night stays at their hotels? I suspect they have looked into it and have decided it isn't really worth the costs.

At some future point, they may up the normal Cat levels for those awards (i.e. Cat. 1-6 rather than Cat. 1-5), but I'm doubtful they would let people "upgrade" those e-certs at will.