I recently purchased a 1979 W123 300TD wagon. The ignition key is different from the doors. I'm not sure but I would assume the ignition has been changed and not the doors. I've read that changing the door lock cylinders to match the ignition key would be fairly easy but how about the reverse? if the doors use the original key I would much rather switch the ignition tumbler so that it would match the original key. Is this doable? Is it a difficult job?