As a follow-up to this thread:
I asked this same question:
- On the AP chat on the website. They confirmed what was said here (you can get in to the park on the day you upgrade)
- Called the AP phone number. They said that as soon as you upgrade you are locked out.
- When I purchased my tickets in person at the gate. The CM said the same as #2 above.
We were nervous about the conflicting information so we ended up getting a 4 day ticket instead so we could just come the next morning before we headed home, but we ended up making it before closing on our last (3rd day). The CM at the booth said it would not have been a problem and in fact said she could get us in the next day (which we did not take advantage of.) And to top it off, they refunded us our fill three days of purchases MaxPasses since we upgraded our newly purchased APs to include MP.