I think I've answered this before, and I know that I've addressed it in my Xenomorph Biology segment, but the queen does
not regrow or reattach her "egg sac" (the proper term is "abdomen").
The queen's abdomen is not an interchangeable object. It is just like an insect's: it is a part of the queen's body (an appendage), and is not something which can be freely reattached.
The queen's abdomen is just like an arm or a leg: it is an appendage from her central torso, and won't grow back just as an arm or a leg wouldn't.
The only reason the queen detached herself in Aliens was because Ripley had already completely destroyed her reproductive capabilities. In essence, she was ripping her body in two.
The queen in AvP seperated herself because Anderson is a frickin idiot.
It is obvious that the queen has no extreme regenerative capabilities because the other xenomorphs do not, either. If they did, Grid would have regrown his tail in AvP, or the cuts on his head would have healed faster.
So, once the queen's abdomen is gone, it's gone for good.
If the queen had to move, chances are she would summon her brood to carry her entire body to a new location.
And thus I clothe my naked villainy
With old odd ends stolen forth from holy writ
And seem a saint when most I play the devil.