Not sure when the plane was switched but it was in the last week for sure since I checked just before I left for London about a week before.

Update on this: I didn't have much time to update since we've been busy but she ended up being back on Delta One albeit at the last minute.

When she arrived at the airport and after she went through security (with the premium economy boarding pass), she checked the app and learned she was downgraded to economy again. So she was in the phone with Delta as she was entering the lounge (Priority Pass). It turns out the plane had switched back to the original one so she was moved to economy again for some reason. In the call, she was finally moved to a Delta One seat.

The flight left 2 hours late and boarding was chaotic. People had a mix of boarding passes and seats were moved around. FAs told people to seat as fast as possible to their assigned seats but I guess they might have changed a bit for some. Luckily, she fixed her problem by calling them beforehand.

In the end, Delta gave her several thousands of miles (plus 3000 due to a problem with in flight entertainment that appears to have gone to a lot of people). Let's hope it doesn't happen on Sunday when she goes back.

For what it's worth, she loved the food, the in-flight service, and the fact that both calls to Delta took about 10 minutes to get her problem fixed.