mega exo and gxp are compatible in terms of spindle diameter, you would need to change the Bottom bracket aka BB from megaexo to GXP. If its a 2013 it may be a BBright with an adapter by WheelsMFG adaptor, So you may only have to change the adaptor and not the BB (
http://wheelsmfg.com/...for-sram-cranks.html )
i would double check with your LBC or cervelo weather you need the adaptor or the BB.
Either way it can work with the proper modification
cheers.