Exactly this ^. I kept my old watch for when I am working around the house, etc. and was able to duplicate most of my passes, cards, etc. except the Rivian key, a couple of my transit passes, and my work access badge. I guess I just wish Rivian had a more informed message when trying to add the...
I just had this issue as I bought a new Apple Watch Ultra 3 but still wanted to keep my Apple Watch 10. After doing some research, it appears this is a Rivian limitation.
Rivian Support - Support Center - Rivian