I believe needing to trade it in is part of the plan requirements. I could be wrong, though. I have T-Mobile and the UP on Demand program is a lease with exchange program, which is fine with me since I don't particularly enjoy the risk and hassle of selling a phone. My old 6 trade-in value more than pays for the balance I have on it so I'm ending up with a credit on my bill. Yes, if I sold it I could get more for it but I don't feel like dealing with selling it. And I like the discount I get with the UP program.
BTW- the T-Mobile signup was kind of glitchy, too.. When they sent me the lease agreement for my digital signature, it had the regular price split up into payments so I didn't want to sign it. I called support and they said the promotional deal would be taken off the bill every month ($7 less). I'll believe it when I see the first bill. There's even a place on the lease agreement for a promotional credit per month and it was 0. Why can't they get their system together to make it more transparent so I don't have to worry about it?? Too many companies in the past have said X will be taken off later, no worries, but then it doesn't happen.