I still owe $500 on a 7 plus. My sister in law wants to buy it. I want to switch to ATT and use a sim-free 8 plus/X on it. Say I order the sim-free phone now, get it in a few days, put my sim from my "unpaid for/locked" T-Mobile phone into the new sim-free phone, will the new phone become locked ? Makes me think of another question: my wife's old 6plus was paid off long before we joined Tmobile. Is it still unlocked? I haven't dealt with this stuff enough. Thanks.