UPDATE: I went in to two T-Mobile Stores with my Moto G (5th Generation) and Moto One, and both phones were able to be updated to T-Mobile. Supposedly, nothing changed with my account. At least that's what the email said. One store had a harder time than another updating my phone, but both eventually worked. However, the phones would not send text messages when I enabled the advanced chat settings, so I've disabled that. And, I still lose reception right near the airport, which means the main problem I had to bother to get on the T-Mobile network was not fixed. But I guess I am officially on T-Mobile with a Kickstart plan, and that's still not bad at $40 a month. Original article follows.
I know I'm not the only one out there with a legacy (year-old) Sprint Kickstart plan. I get two accounts of "unlimited" data and Sprint's blah network for $38 a month total. I think it was $15 per line plus taxes and fees. That's not a lot of money for two mostly reliable phones that can use as much (throttled) data as I want, so I want to keep it.