If Find my Apple Watch is enabled, you’ll need to enter your Apple ID password in order to unpair the Watch.
Wait for the Apple Watch to unpair. This may take upwards of half an hour depending on how much data is on your Apple Watch so make sure you make the time and have adequate charge on your Watch.
You’ll see an Apple logo with a ring around it on your Apple Watch, indicating that it is unpairing from your iPhone. You may see this screen several times as the Watch is unpairing. To confirm that the Apple Watch is completely unpaired, you’ll see a Welcome to Apple Watch screen on the Watch itself.
Once unpaired, go to your new iPhone, head into the Apple Watch app, and tap Start Pairing. If you’re on iOS 11, a card will automatically appear on your iPhone’s screen requesting to pair. Simply tap continue to start pairing.
You’ll be prompted to hover your iPhone over your Apple Watch with a visual QR code displayed on the Watch’s screen. You’ll then be asked to setup your Apple Watch as new or from backup, choose restore from backup and select your most recent Apple Watch backup.
From there, follow the on-screen steps on your iPhone. This includes enabling Siri, Apple Pay, and entering in your optional passcode.
Now you’ll see the Apple logo on your Apple Watch with a ring around it. You’ll have to wait until the ring is filled and then Apple Watch setup is complete.
While it may seem like a lot, unparing your Apple Watch is fairly easily once you’ve done it a few times.