# cd /etc/zypp/repos.d
# sed -i 's/11.4/12.1/' *
# zypper ref
# zypper -v dup --no-recommends
The only small glitch I had was systemd apparently taking a nap during boot, which made the machine take very long to boot (about 2 minutes).
After filing bug 730776, I found out (with the helpful hint from Frederic), that I still had an invalid swap partition in my fstab. After fixing that, the bootup now really is blazingly fast:
server:~ # systemd-analyze
Startup finished in 4116ms (kernel) + 33827ms (userspace) = 37944ms