You will for the most part remain in the underground platform area of Frankfurt's main station. It's perfectly safe 24/7 IMO. Lots of security cameras and security personnel there.

I wouldn't recommend wandering into the more deserted areas of level B (that's the level on which you switch platforms). But you have no reason to go there. You just need to take an escalator up to level B and take an adjacent elevator back down to the other platform...