We were in the parks Monday and tried to add the $75 option for the remaining 9 months of our Signature APs, had read on Disney's site we had to do this at the ticket booth.
Ticket line was very long when we arrived, so decided to just go back and do it later. After a fun morning, it started to get crowded so we stopped by DCA Guest Services hoping we could avoid the ticket line, but CM told us they couldn't do it there.
After 30+ minutes in the ticket booth line the CM apologized for the wait, but told us they were unable to do it because of a computer system issue and they were hoping they'd be able to later in the day. She instructed us to go ahead and pay the $10 for the single day MaxPass in the official app, save that email receipt to show at the booth later and they would credit us for it when they were able to add the $75 MP to our APs.
Saw someone in this thread mention they were able to add the $75 option by calling the AP phone number, so tried that hoping to avoid the ticket booth, but CM said they couldn't give us the credit for the single day passes and to just go to the ticket booth next time.
Despite all this, we still had a fun day at the parks and really enjoyed MaxPass once we got it. We were able to do 16 rides (including GOTG twice), saw a parade, fireworks and got incredible free spots right next to the BB premium area for the new Fantasmic