What do i do if the cement board doesn't line up with the exiting drywall?

my fibergl shower pan was cracked and my tiles were coming off. so I took out the tiles and greenboard, and replaced the shower pan. unfortunately, they do not carry the same size of shower pan - I am short about 1/4 inch on all sides. what do I do in order to line up the cement board with the existing drywall? i only plan to replace the area where the old tiles were. i am afraid I have to tear down all the drywall but I really don't want to. thanks.