Interesting -- this is a good development.
I've been a AT&T UDP guy for 7 years and I still am, at least for a few more weeks. I just decided to switch over to T-Mobile and I'm waiting on my 6s+ to arrive. When it does, my plan is to port over my AT&T number and cancel service with AT&T.
For years, I've been saying that I will never give up my UDP. However, after thoroughly crunching the numbers from every angle, I've concluded that it just isn't worth it for our (my wife and me's) particular case. By switching over to T-Mobile, we each get 10 GB of high-speed data a month and music streaming is always free. I can also freely tether as much of that 10 GB as I want, for free. I've never used more than 5 GB in a month and my average is 2-3 GB a month. I'm fully aware that data usage will continue to grow over time.
After running the math and carefully considering every single variable, I am saving $31.50 a month by giving up my AT&T UDP and going with T-Mobile. That comes out to a savings of $378 a year! And when you add up that savings year after year, the savings are huge.
Sure, signal coverage is an unknown until I actually get out there and try it out, but T-Mobile has made tremendous advancements over the last year and continues to do so. I suspect that the coverage differences will be somewhat minimal for the region I'm in.
So, even though I've been a UDP customer for years, I finally did the math and it just doesn't make sense to keep a death grip on something that is actually significantly more expensive in the end. I am super stoked though to see that AT&T is changing their policy. It's great for existing UDP customers and it's the right thing to do.