Linux in-place upgrade

Hi,

We are planning an in-place upgrade of Linux level from RHEL 7 to RHEL 8 or RHEL 9. We have several PG clusters running on this server using different versions of postgres (pg12, pg13 and pg14).

We have couple of plans:

1. Shutdown clusters, unmount disks that contain postgres data and wals, upgrade Linux, remount postgres disks and restart postgres clusters
2. Same as above but reinstall postgres (pg12, pg13 and pg14) before restarting postgres clusters

Questions:

1. Will 1 or 2 or both above work?
2. Is there any official documentation above OS upgrades?

Thank you for reading my post.