Unity 6 Preview is just the name for releases in the Unity 6 stream that are currently available. It’s not like these are fundamentally different things; Unity 6’s full release is just going to be the releases that are available from Oct 17, pretty much just removing the word Preview. Features are already locked in by this point, updates focus on bug fixes and polish. You’ll have less fuss upgrading a couple versions ahead vs. upgrading from a completely different stream of releases like 2022.3.