This blog post should first serve as a public service announcement: Before you sell, erase, lose or otherwise divest yourself of your current iPhone and move to a new iPhone, Apple Watch wearers should unpair their Apple Watch from their old iPhone. Because if you don't, the only way to pair it with your new is to reset your Apple Watch and erase everything in the process.
If your old iPhone is no longer in the picture, then you will need to return your Apple Watch to its original factory settings before you can pair it with your new iPhone. Here's how:
On your Apple Watch, head to Settings > General > Reset. On the Reset screen, tap Erase All Content and Settings and then tap Continue to erase all media, data and settings.
After your Apple Watch resets, you will need to choose a language and then you can pair your Apple Watch with your new iPhone, either by using the camera or manually.
After hopping through a number of setup screens, starting with agreeing to terms and conditions and choosing your wrist preference, you will be given the option to install any apps currently installed on your iPhone that are compatible with the Apple Watch.
So, even though you need to reset your iPhone, all is not lost since you can quickly sync a number of watch apps that you installed on your new iPhone.
And if you still have your old iPhone, here's how unpair your Apple Watch: Open the Watch app on your iPhone, tap the My Watch tab and then tap Apple Watch at the top. Next, tap Unpair Apple Watch and tap again to confirm.