Did you have the mizzen set? I haven't sailed a ketch but I would imagine that would make a big difference. The new main probably does have the centre of effort much farther forward than the old main. With the new main being flatter you can probably sail closer to the wind and you may find you need to trim the mizzen closer to centre than with the old main. Can you tell if your board is fully down? Try loosening the main halyard as well as the out haul, this will move the centre of effort aft a bit. You probably don't adjust your rig very often but just in case, make sure you don't have too much mast bend. You could try raking the mast aft if you still have lee helm.