When you pair your Apple Watch with your iPhone, it becomes associated with that specific piece of hardware, so when transferring your data to a new iPhone, your Apple Watch pairing doesn’t actually transfer with it. Unfortunately, while Apple does have a support article on the subject, you have to actually know to look for it — Apple doesn’t make it proactively clear that there are steps you should take before you decide to transfer your data to a new iPhone to avoid the risk of leaving your Apple Watch behind.
The problem is that when you switch to a new iPhone, you will have to re-pair your Apple Watch with the new iPhone, setting it up as a brand new Apple Watch and then restoring it from a backup stored on your iPhone. While the Apple Watch does get backed up to your iPhone on a regular basis — and these backups are included in your iPhone backups stored in iCloud or iTunes — due to the frequency with which your Apple Watch tracks things like activity data, there’s a good chance that your last backup won’t be current, leaving you with the possibility of losing your morning workout data, for example. Sadly, iOS does not yet provide a way to force an Apple Watch backup to run on demand.
So, before upgrading your iPhone, Apple’s recommendation is that you first UN-pair your Apple Watch with your old iPhone by going into your iPhone Watch app, tapping your watch at the top of the screen, tapping the “i” to the right of your watch, and then tapping “Unpair Apple Watch.” This will unpair your Apple Watch and perform one last backup of your Apple Watch data in the process.
Once you’ve done this, manually perform one final backup to iCloud or to an iTunes encrypted backup. This is the backup that you will restore to your new iPhone.
After your new iPhone is set up and restored from backup, you can then pair your Apple Watch with your new iPhone by going into the Watch app and going through the usual Apple Watch set up process. During this process, you’ll be asked to select a backup to restore to your Apple Watch, and once this completes your Apple Watch should be ready to go.
Note that it may take some time to restore all of your health and activity data from iCloud, so don’t be too concerned if your Activity Rings don’t look filled in right away — they’ll catch up eventually. It also appears that going into the Activity app on your iPhone will speed things up a bit, as the iOS 11 restore process seems to prioritize restoring data fro iCloud for apps that are actually in use.