I'm trying to get the free dining deal this fall. I'd like to get the fewest days necessity with my resort package and then turn those days into an annual pass once we arrive. Does anyone know if we would be able to update the magic bands that we receive with our package to work with our APs?
Throwing out the MB issue (which is simply not a problem, as explained above,)
The situation about which you are asking DOES actually have a limitation for you.
If you advance-buy only a 2-day ticket (Hopper, I believe) which is the required minimum ticket for Free Dining, then
you can only book advance FP+ for 3 days of your upcoming trip.
If you intend to book a longer trip with more days in the theme parks, it can be wise
to purchase enough days on your original package tickets to cover your entire first trip.
Any money that you pay for those tickets will be "saved/recovered" when you do the upgrade to AP,
and you will be getting the ability to book advance FP for the ENTIRE TRIP as early as 60 days
before check-in at the resort.
So, you will be gaining a lot of convenience (some would say "necessary advantage")
buy booking more "days" on your tickets... and not paying anything "extra" to do so.
The only difference is WHEN you would pay for that part of the cost of your new AP.