The SDB:System upgrade uses --releasever, but that only works if the repo
definitions have the $releasever in the correct place. This is the case in
newer installs (at least a fresh 15.1 gave me that), but I have also
installations that are originaly done fresh with 42.n. They where ugraded using
the "change the repo definitions and run zypper dup" method until now, but they
never got the $releasever strings.

IMO the documentation should mention that this only works wit repos having
$releasever in their URI and thus must be edited before --releasever can be
used.