Since i'm on it to migrate my fifth machine from systemV to systemd (with an delay of over one week per machine) and i've searched now the third time a the solution for the following step, i want to provide my results here.
The debian wiki provides a description for this task when you are using grub2.
# $EDITOR /etc/default/grub GRUBCMDLINELINUX_DEFAULT="quiet init=/lib/systemd/systemd" <--- Change this line
or (for arch linux)
grub-mkconfig -o /boot/grub/grub.cfg