Comment # 6 on bug 1021969 from Franck Bui

(In reply to Tony Mechelynck from comment #4)
> 
> You might call this a question of principle: without rebooting, the newly
> installed systemd (or sysvinit) executable would never be run AFAIK.

Please have look at the systemctl man page, specially the "daemon-reexec"
command.