Just thought I'd add our recent experience as an FYI so others can watch out.
Background: I upgraded DD14's 3 day hopper to a Flex Pass in the app when we were done in the parks on our last day, 5/27. I tried to pick up her physical card at the ticket booth but they wouldn't let me since she wasn't there (she and her friends were at Pop Up Disney). I was a little annoyed but whatever. She said something about verifying the photo but that didn't really make sense to me since I wasn't trying to change the photo or anything and her name was already assigned to the ticket. We needed to start the drive home so we decided we'd pick up her pass at our next visit, which I had already booked the reservation for (6/20 &21). I did notice that the expiration date of her Flex AP was 5/24/20 which was as expected since we first used the 3 day PH on 5/25/19.
Fast forward to 6/20. DD14 went to the ticket booth after we left DCA that night to pick up her physical card. I didn't go with her but another adult friend was there. The CM asked her if that day had been her first day in the park with the ticket/pass and she said no, it was a previous visit but she didn't specify the date (and the CM didn't ask any further questions). CM gave her the card and off they went.
The next morning, I randomly noticed in the app that her pass was now showing that it would expire on 1/6/2020 (which would correspond to when I purchased the 3-day PH that we upgraded to the Flex Pass). Obviously this makes no sense since the Flex Pass wasn't even available in January and it would not have been possible for me to upgrade a PH to the Flex Pass at that time. We had a full day planned at the parks so I tried not to stress too much and planned to stop by the ticket booths on our way out that evening.
When I got to the ticket booth, by total chance I got the same CM who had helped my daughter the night before (DD14 has a relatively unusual first name) and remembered the conversation about the first day. She said she didn't know the first date of use (but remember- CM also didn't ask DD14 the date or indicate to her that it was important!) so she used the purchase date but could adjust it. It was pretty painless in the end to get it fixed except that it did involve the extra trip to the ticket booth. I feel like it could have been a lot more of a problem if I hadn't had the same CM when I went to get it resolved. Lesson learned for DD14 that sometimes follow up questions are needed but I wish the CM had been more clear with her about what she was asking/why she needed the information as well.
TL;DR; make sure your expiration date still looks good after you pick up your physical pass!