Thanks for the knowledge! Early on, like with the first Apple Watch, I thought it was different. But I will remove the plan when I unpair the watch.I just went through this. When you KEEP your cellular plan, it STAYS with that watch. You have to REMOVE the cellular plan and then add it again (via carrier QR code, or manually) on your new watch. Also, when unpairing the watch it will do a back up. When you restore to the new watch, it should ask you if you wanted to add the eSIM to as well. If not just do it manually like above.
Removing a cellular plan from an Apple Watch does not cancel your service with your carrier.