Changing smart watch on plan

Going from an apple watch ultra 2 to ultra 4. I have the free companion watch plan on warp. So looking at the USMobile help center it talks about removing the plan, syncing the new watch to my phone, then adding the plan.....but looking at the usmobile app, there is a switch watch selection and I can enter the IMEI number of the new watch....which one of these routes should I go?