The difference is that in place upgrades aren't possible for major versions.
Re:PostgreSQL 15
by jeff-davis on Sunday October 16, 2022 @01:35PM
(#33225463)
pg_upgrade is the way to do in-place upgrade between major versions:
The only change is that versions used to be X.Y.Z, and now it just used X.Y. Now, X gets incremented for every major version, so it seems like it's moving faster.