I would think the TA could just make the change. I always book directly with Disney so I don't have any experience with a TA. But we booked the Deluxe Plan for our December trip and I asked if it would be ok if we ended up having to downgrade it. I was told no problem up until 45 days out because of a downgrade. I am pretty sure you can upgrade up until a few days, maybe a week before your trip.