What happens to the portions of large glued displays that aren't retasked into a different display?
In most cases they would be reused in other parks, donated to LEGO retail stores, auctioned for charity or hidden away from park visitors. As a last resort, LEGO models can and will be recycled to create new bricks. 

The LEGO auction site, Bricklink has a fairly hidden place to trade glued models and retail displays.