You can now self activate t-mobile SIM to TNX program from Sprint.com website. I have successfully activated it on Samsung Galaxy A71 today using t-mobile R15 sim card. Download t-mobile App experience
Replacing a Sprint SIM card - Calyx Institute
6 WEEKS to SWAP Sprint for T-Mobile SIM Cards – C.D. REIMER
T-Mobile is paying customers to swap out their Sprint SIM cards