I have At&t, I unpaired my watch, during that it asks if you want to keep or remove your cellular plan from the watch. I removed it, and paired the new watch, asked for my carrier details and it added it to my new watch. Worked easily for me, I know others have had some issues.
Yes it gives the option to transfer number or get new, i’m on Verizon so can’t comment on other carriers. One thing for don’t call Verizon they haven’t a clue, again I would have thought they would have learned from the series 3 debacle.
The post above mine is what I did. Verizon is having severe outage problems today, customer service is down also. Might wait until they fix their issues, it might be affecting activations.