From what I understand, as of right now the only option is to purchase once you activate your tickets at the turnstile. If there is an option to upgrade onsite the day of, you would have to forfeit any Magic Morning because they would then no longer consider your tickets to be pre-purchased. You might also lose out on any discount you got from a vendor.
I know that some have had a card loaded to their account ready to purchase the MaxPass so that might be a tip. We have only purchased MaxPass once and that day it was super easy to purchase it so while there have been hiccups, I believe there have also been many more days that it works correctly. What I would suggest is to have the very first step of your mornings be very flexible in case you run into any hiccups (ie no dashes to Peter Pan).