How can i get back my old phone trade program?

I traded my ip11 pro max to ip14 pro max. I payed off new ip14. Now, i want get back ip11 because I don’t want update my plan, the cost for new plan go up a lot because i have 3 lines in my current plan. They need me update all 3 lines to unlimited plan in trade phone program for only 1 new phone. That so expensive