Trade in "Good Condition"?

“You're welcome to Intercambia tu dispositivo at one of our T-Mobile store if you'd like to have our retail team look at it. You can also get an estimate using our Intercambio de celulares | Intercambia tu teléfono usado por uno nuevo | T-Mobile page before you make the trip.”

where can I find/read the fine print regarding the ‘qualified condition” for an upgrade/trade in?

I'd advise you not to trade in with T-Mobile. Any defect at all and they give you nothing but they still keep the phone. Better to get it unlocked and sell it on Swappa.com. At least then if there's a disagreement, you get the phone back.

This is kind of what I'm afraid of.  I heard that Samsung was doing something similar when you traded in with them directly, and the stories have made me very cautious about just sending off a phone blindly.

I'm not saying that it will necessarily turn out that way but it does often enough to make me doubly cautious. The thing that's terribly unfair, is that when they decide that they don't like something about your phone, they don't send it back, they keep it and give you nothing. You have no recourse, no appeal. What you get is entirely at their discretion. I just don't like the idea of doing business that way. At least with Swappa, you should get something and you have recourse. Just get the phone unlocked first.