Who's talking about upgrades at Grande Vista?
In general, most timeshare resorts will not be able to upgrade you---every room in every building is spoken for by some owner, so there's no empty unit to upgrade you to. Some resorts might offer low-cost upgrades as a sales tool for unsold inventory. For example, I believe Silver Lake has done this from time to time. However, I don't believe Marriott does.
That doesn't mean you shouldn't ask. When you check in, ask if it is possible to move to a larger unit, and what the cost for that might be. Don't act as though you will expect it to be free, or even cheap---that's one guaranteed way to signal to the desk clerk that you're trying to freeload

Instead, act as though you fully expect to cost and cost a lot. If they happen to have a program in place for doing this, this is your best chance at getting a fair offer.
If they offer, and the price is to your liking, go for it. If they offer, but the price is too high, politely decline the offer---"Hmm. Interesting, but that's a bit beyond my budget." You never know if that will generate a lower offer or not.