Two weeks ago my phone 6s was smashed at work. I put my SIM in my iPhone 6 and restored from backup. I’m fairly certain that I had to erase my watch manually and then pair to the new phone and was then prompted to restore from backup. My replacement phone came today and I just set it up using iOS’s automatic setup and restored from the iPhone 6 backup. An Apple Watch pairing prompt opened an asked if I’d like the watch paired to this phone in which I choose yes. I set the phone down to let it restore and a short while later my watch started ringing with the following notification. I scrolled down and selected yes. My watch did a quick reset, load, and came back up to the watch face and unlocked like nothing happened. I just verified that the watch is no longer paired on my old phone so I guess all is well.