Just bit the bullet and dialed 1-800-XFINITY, as instructed in the emails, to schedule the installation of the "upgraded" X1 boxes.
Apparently this isn't something you can just exchange for your old boxes at the local store. They come out to install, and to make sure everything's working as it should. And it's at no charge, so I can't argue with any of that. The worst part of these things is having to endure the phone call with someone on another planet, and the endless repetition of every aspect of the upgrade deal. The people who come out are usually great.
So, Saturday morning between 8 and 10. I could have had that slot tomorrow, but - wouldn't you know - I have rehearsal for the last of my spring gigs tomorrow morning, and Saturday was their next available date.