Schmeck is referencing a rule that says that a ticket cannot be upgraded
on a day AFTER all of its assets have been used up (even if it's still within the 14-day window.)
But, that rule does not apply to guests staying at a WDW resort.
Since you have a MagicBand (at this time) it means that you ARE staying at a WDW resort (because they are only issuing MB to resort guests at this time)
so you don't have to worry about that rule.
OTOH-
Yes, you CAN put an AP on an MagicBand, so you don't need to
wait until the end of your trip to do the upgrade.
(But, you COULD, if you wanted to.)