thanks for the response guys.. basically you have to unpair the watch first from your existing phone so you can re-pair it with another one. In my case, I didn't unpair is so that means I had to restore my watch before it becomes available for re-pairing again. It's now paired to the 5s. now I had to do this thing all over again for the 6s plus.
suggestion: when you un-pair, that's when the watch backs up to the paired phone. so if I unpaired my watch to my 6 plus, the watch will create a back up to the phone.
unpair/watch backup > old phone backup > new phone.