"Rolling forward" aka 'guix system reconfigure' can typically be done without a reboot (but you need to restart affected services manually -- Guix will tell you which).

"Rolling backward" aka 'guix system roll-back' is not as integrated IIRC and is easiest with a reboot. You can however "roll forward" to a previous generation (in this example generation 117):

  cd /var/guix/profiles/system-117-link
  guix time-machine -C channels.scm -- reconfigure configuration.scm