Comment # 11 on bug 1223167 from James Carlson

(In reply to Stefan Hundhammer from comment #10)
> Actually, that should work. The upgrade using DHCP to reach any remote repos
> is also the default behavior. 
> 
> This leaves the question if your 15.4 had that symlink
> /etc/products/baseproduct (see comment #7) to check what existing system to
> upgrade from. I am not even sure if it got that far in your case; I'll check
> again.
> 
> Let me have another look and ask around in the team.

I don't see any /etc/products/baseproduct, but there is an
/etc/products.d/baseproduct:

% ls -sal /etc/products*
total 24
 4 drwxr-xr-x   2 root root  4096 Jul  8  2022 .
12 drwxr-xr-x 175 root root 12288 May 22 08:00 ..
 0 lrwxrwxrwx   1 root root     9 Jul  8  2022 baseproduct -> Leap.prod
 8 -rw-r--r--   1 root root  4210 Jul  4  2022 Leap.prod

I'll attach a copy of that Leap.prod file.