If the phone is already unlocked then yes it should work with any sim in it (as long as it's a GSM phone). You don't need to unlock an unlocked phone again. T-mobile should switch things over regardless, yes.

Hello everyone,

I tried searching but for some reason, couldn't find an answer to all of my questions. If there's already a thread I overlooked, I do apologize!

My girlfriend recently broke her phone and she is with T-Mobile (in the USA). If she purchases an unlocked phone, can I simply change the SIM cards and then follow the unlock internet thread and everything will work?

Also, if she buys a T-Mobile phone (a used one) and takes it in, will they activate it for her/switch over her plan for her?

Thanks for your help. Again, I am sorry if this is a duplicate thread (and actually a bit surprised I didn't find this very question being asked).