Will upgrading kill my remaining bill credits? (17 Pro, ~12 months in)

On T-Mobile, Go5g plus account. I upgraded my 15 pro to an iPhone 17 Pro last September on a 24-month promo with monthly bill credits. I'm about 12 months in, so roughly half the credits are still outstanding. I did not upgrade any of my other lines. I want the 18 Pro when it drops. Two questions: If I just trade the 17 Pro in on that same line, I lose the remaining credits and still owe the EIP balance, right? My understanding is the no-early-payoff rule applies since I financed after July 2024. Instead, can I upgrade the 18 Pro on one of my other lines, leave the 17 Pro EIP alone, and then transfer eSIMs so my number ends up on the new phone? Does that keep the 17 Pro credits running, or does T-Mobile care which IMEI is on the line? Anyone done the second route recently? Any gotchas?