Last year when I migrated from S3 to S4 on EE all I needed to do was:
- Unpair S3
- Select ‘keep plan’
- Pair new watch (based on back up, but i’m not sure that is necessary)
- Confirm that you want to apply plan to new watch.
Now... that didn’t actually work smoothly. The plan should migrate within the hour, but even after overnight it hadn’t. At that point I contacted EE customer service and they took the EMEI of my new Watch to sort out the connection. Hopefully this year it will be smoother for people making the change (I haven’t decided if to upgrade yet, waiting on a friend’s review before committing).