Re-add or Pair an Android Wear Watch without a Factory Reset
Note: If you cannot connect your watch over a USB interface (such as an LG Watch Style), make sure it's connected to your WiFi network before resetting/changing your phone.
Step 1: Enable ADB Debugging.
Go to Settings --> System --> About, and tap the Build Number until it says "You are now a developer."
Then, back out of the About page and tap "Developer Options". In there, enable ADB Debugging.
For watches without a USB interface, enable Debugging over WiFi.
Step 2: Open Command Prompt or Terminal on your computer
Open Command Prompt (Windows) or Terminal (macOS) and type in the follow command:
adb connect watch's IP address:5555
adb connect 192.168.0.255:5555
Step 3: Issuing the command
Now type in:
adb shell "pm clear com.google.android.gms && reboot"
Once the watch reboots, type in the following command:
adb shell "am start -a android.bluetooth.adapter.action.REQUEST_DISCOVERABLE"
A message will appear on the watch asking if you want it to become discoverable. Select the checkmark and open the WearOS app on your phone, and set it up like a new watch.
All is done!
Enjoy your now connected watch without having to set it up again.
Source (thanks XDA!): https://www.xda-developers.com/pair-android-wear-without-factory-reset/