I walked into the SFO United Club today, and as usual, I gave them my boarding pass to scan for my membership. Usually this is a two second process, but the agent asked me if I was a member. I said that I was, and she said she needed my Mileage Plus number. I did my best to try and remember my new number, but she stopped me and said, "No... the old one..." I remembered that one.
After I sat down, I logged into the T-Mobile WiFi. Once again, I tried my new number. No go. I tried the old one. Success.
I'm sure there will be similar cases of this over the next couple weeks. It looks like it's not quite time to un-remember that old Mileage Plus number yet!