Yep I sold my AW4 to a mate. When I unpaired I said "keep plan", but now this plan is on my new AW5, so my mate can use cellular on it if he wants (he doesn't as it is as he is on 3)
Yes, I am talking about the Watch app, when you go to Mobile Data it said something like
DATA PLAN
EE (i)
Mobile Data (i)
Unused Plans
EE (i)
I clicked the (i) after unused plans and had the option of 'new plan' and 'remove plan' from memory, I hit new plan and this then activated and removed the unused if you get what i mean. I have confirmed in the EE app that I only have one data plan for the watch and it is the £5/month one