You are on OVH, have gone with the default layout on a box, and now realise you'd rather mount differently.

remount.md

1. Make sure you allow root to ssh in: Edit your /etc/ssh/sshd_config, add your public key to root's authorised keys, and so on.
2. Move home, e.g. mv /home /home.bak.
3. Comment out the line that mounts home in /etc/fstab.
4. Reboot.
5. Rename home.bak back to home.
6. Stop PostgreSQL.
7. mv /var/lib/postgresql /var/lib/postgresql.bak.
8. Edit /etc/fstab, remove comment and replace /home with /var/lib/postgresql.
9. Create /var/lib/postgresql with chown it to postgres:postgres.
10. Mount it.
11. Move contents of postgresql.bak to postgresql.
12. Start PostgreSQL.
13. Clean up after yourself: Revert your ssh config, remove the key you've added to root, etc.

I lied in #2. I actually copied cp -Rp /home /home.bak and was trying to simplify in the writeup. @ezkl pointed out mv doesn't work.