Thank you for this great tutorial, just made my update.
One thing perhaps, I had $releasever in my repo files instead of a hardcoded version on my home server.
In order to be sure releasever gets the new version on zypper call, I would recommand to add --releasever=15.6 to the zypper calls, which would be ignored on hardcoded repo files, so it's a win-win.
Also, perhaps one could check that easily forgotten services (I think grommunio-archive) is still working, eg:
[ -d /etc/grommunio-archive ] && ss -laputen | grep $(grep archive /etc/postfix/transport | awk -F':' '{print $3}')
Again, thank you, this really makes the upgrade a lesser buren.
