The motherboard is actually mounted in the case with a series of screws through the motherboard, seven in this case, all of which must be removed. About the worst thing that can happen when you're replacing a motherboard is that one of the screws will turn and turn without releasing. Normally, this is due to the screw having been over-tightened in a brass standoff, which comes unscrewed from the motherboard pan and remains attached to the screw. If you think this is happening, proceed to removing the rest of the screws first so you won't place undo strain on the motherboard by flexing it up. If the standoff thread in the motherboard pan is stripped, you can take off the other side of the case and grab it with vise grips from the back.
