Yes you should be able to upgrade. Remember if you are only upgrading for the food discounts you have to get one of the 2 top tier passes. The cheaper passes don’t get food discounts in the parks or city walk. But may still get a discount at the restaurants in the hotels.
Although we have gotten discounts on food with our seasonal passes during pass holder appreciation so if you are going during that time a cheaper pass may work,
If you have a larger group going it may still be worth it to upgrade for the dining discounts as you only need 1 person to have the pass (as long as they are always at the check out to show it).