Thanks for your help. I guess I’ll have to go tomorrow to see if I can do it in person. Do I have to make a reservation for tomorrow or can I just apply an unused ticket to a magic key at the ticket booth?
Both answers hinge on whether the Imagine will still be available tomorrow. I assume it will be - it lasted a few days last year - but, obviously, there's that gamble there.
Doing it at a ticket booth, you wouldn't need a park reservation. It'd also have the advantage that you wouldn't have to activate the pass tomorrow - you could save it for your visit next month, and the year would start that day.
But if the booth line is super-long, and/or you don't have time to wait - or if you wanted to visit the park tomorrow anyway - you can get a park reservation, enter the park, and upgrade on the app. Keep in mind - if you do that, the pass would expire a year from tomorrow. The big upside is that you could get there at park open and be done five minutes later - you won't have to wait for the booth to open (which, if they do it like today, might not happen until 9am).
I'll add this since I've never done it - someone else will have to chime in as to whether you can do the monthly payment plan on a ticket upgrade in the app.
One more FYI - only because you said "tickets" - you can only upgrade one ticket to an MK using either method - you can't combine multiple tickets. (The app trick would only work for your ticket, too - if you had a ticket for someone else, they'd have to come with you and enter the park to upgrade in the app.)