What will harden up? Do you mean the honey will crystallize? Have you drained the Flow frames? Was the honey in the Flow frames capped? Was there any honey in the Flow frames? Need more info to answer this accurately.
However, worst case scenario - you have hard crystallized honey in the frames. Just soak them in tap hot water (less than 70 Celsius is safe for the plastic and hotter than most household water heaters). The wax will melt and the honey will dissolve over 15 mins or so. Easy.
Even better, don’t do that. If the honey crystallizes, just give the frames back to the bees next season, after freezing them this year. That way you kill off any wax moth and SHB larvae, and you recycle the honey. The bees usually clean the old honey out of the frames before reusing them, just like they do with traditional wax frames. 