I had 2 iPhone upgrade plans and was able to swap the sim chip in them with no problems. I always got the unlocked iPhone tho (with the upgrade plan). It’s linked to your Apple ID in my experience.
The 12 PM was the first time I left the Apple Upgrade Plan and did the 24 months Apple Card payment plan. I’m now on a 13 PM - (always buying unlocked) - still paying off the mother-in-law’s iPhone via an Apple Upgrade Plan (will be paid off this year).