My mom wants the iPhone XS Max. She currently has an iPhone X from T-Mobile. She purchased it with a payment plan and traded in her iPhone 6+, for which she received $300 in monthly credits - over 24 months. She wants to buy the Max and give the X to my dad, so he can use it on his existing T-Mobile line. Since she is still receiving the monthly credits towards the X, would switching the X to another line affect anything (cancelling future credits)? She is not cancelling the line, so I'm not sure how T-Mobile would view switching the phone to another line.