I have a 503a and it has the "top hat" cams. they're super. with mine you move the width to straight stitch and remove/install a new cam. the stitches are just a little fun extra. i always have a cam in, but if one isn't in, you can only do a straight stitch.
the link in your last post didn't work for me, but i think you mean that your cams are under that little trap door i was referring to.