The cleanest and most secure way to do this without losing your current plan and selling your old watch without any issues is
1) Wait for your new watch to arrive
2) Unpair your current watch but keep the plan (this will create a current back up)
3) Re-pair your current watch with your current plan
4) Using the back up, set up your new watch ensuring that WiFi is turned off on your phone
5) When the screen comes up asking to connect your data plan, it will take you to a white screen.
6) Call EE and ask them to transfer the plan to your new watch - They will ask for the EID number
7) Reboot the watch and within an hour or two (normally), your new watch will have your transferred data plan
8. Don't unpair your old watch but instead, use the settings on your watch to factory reset the old watch and delete the plan when it asks you. This will remove it completely for resale
9) Log on to the cloud using a PC or Mac and ensure that your new watch is listed and if your old watch is still on there then delete it. This will ensure that activation lock is disabled so you can sell it on
