21 year customer! Can’t change my plan without losing current phone promo

Been a T-Mobile customer for 21 years, T-Mobile lies about offering existing customers the same promotions as new customers! I can't switch plans without lt costing me $50 a month in lost phone trade in credits? I pay $320 a month, I have 4 lines, 2 of them have phones that are owned and the other 2 have phones on installment plans that have a $25 per month credit per installment for the phones we traded in that were already paid off. So basically if I change plans to save money, they want to charge me full price for these 2 phones, then send me back the 2 iPhone 12s that were traded in.