Don't flip them up, rotate them to the other side by swinging them along the top axis. this puts less stress/rotational inertia and velocity than a flip will. if you have or are using a topbar, you do not even do that usually. It will flip them upside down and vise versa. the bees themselves can of course go upside down no problem...but eggs, even though 'glued' in could potentially come lose, or bees could just lose their grip/etc...not to mention of course if the comb comes undone then you are potentially in for a real mess. That being said I am sure everyone has done it, probably, and will come out of the woodwork and say so.