I hope that's not the case, as I spoke to multiple reps before today verifying that we could switch plans and be eligible.
I had read somewhere (not sure if here or Reddit) that phones on plans prior to going to Go5g Next do not count towards the "Next" feature. So you'd have to pay that phone off, then the first phone you get on your Go5g Next plan is eligible to be traded in the year after (once it's 50% paid off).
It's interesting, but when I reviewed the plan terms a few days ago that restriction wasn't mentioned. When I went ot check the terms docs earlier this morning while trying to get this done, they mysteriously weren't available. Just now, they are availble again and that restriction is now right there on the second or third sentence. Sounds like someone at Tmobile didn't think through what might happen on iPhone preorder day and are now scrambling to cover their asses and avoid a mountain of bait-and-switches.