Debian 7 has a fairly old version of systemd, version 44, and when it was released there hadn't been much work done to integrate it into the system. The backports repository has the more recent version 204. Instructions for enabling the backport repository are here:
In theory, it should be as simple as installing the correct package and rebooting:
apt-get -t wheezy-backports install systemd-sysv
Though, I'd highly encourage taking a snapshot and testing it on that first.
More info on systemd in Debian can be found here: