Yeah, apparently if you're playing on PS5, you must not do the upgrade and then you can still play with other PS4 users using that version on the new console. Makes sense because the PS5 version has some extra elements such as new animations, celebrations and other gameplay factors so it wouldn't be technically possible to cross-gen this way. PS5 version is basically a different game whereas most other PS4 titles that you can 'upgrade' just benefit visually or in performance rather than in-game aspects.