Just received a replacement iPhone 6s for my wife and during the setup there is a screen that says 'choose carrier'. the phone that we currently have was originally t-mobile iPhone 6s, but it was unlocked and we use it on ATT network now. The thing is, we actually planning to switch to Verizon in the next 1-2 months and would like to bring this phone over there. If I pick ATT now, will this phone be locked to ATT, or can I pick up Verizon and still use it on ATT network? I appreciate any assistance with this.