As I recall, and its going back a few years to uni. days, the centre of lift does move back and forth as speed varies. A notable example is Concorde where there is huge change subsonic/supersonic so it needed to shift fuel to compensate. The idea of a fixed centre is to make the maths simpler. We fix the centre of lift and have a variable moment - twisitng force - about the same point. So all the forces act at this one point simplifying the equations. For full details you'll need to check a textbook.