Comment # 1 on bug 899506 from Petr Gajdos

I tried with plain rpm -Uhv to go from 18.1.2 to 25.1 without success in
reproducing this issue. Even zypper dup works on changeroot with /etc/HOSTNAME
change and also unchanged.

Michael, would you have an idea what's wrong? /etc/HOSTNAME was regular file
and now it is symbolic link to /etc/hostname.

Relevant parts of scriptlets:

%pre
if [ "0$1" -ge "2" ]; then
  if [ -f /etc/HOSTNAME -a ! -L /etc/HOSTNAME ]; then
    if [ -f /etc/hostname ]; then
      mv /etc/hostname /etc/hostname.rpmsave
    fi
    ln -f /etc/HOSTNAME /etc/HOSTNAME.rpmsave
    rm /etc/HOSTNAME
  fi
fi

%post
if [ -f /etc/HOSTNAME.rpmsave -a ! -L /etc/HOSTNAME.rpmsave ]; then
  cp -a /etc/HOSTNAME.rpmsave /etc/hostname
fi

Discussion is in yet referenced bug 883999.