You'll most likely need a spacer. You'd want to measure the depth of the reflector over the light engine, sometimes it will extend below the light engine. Also measure the optic to see what the difference is, any difference will need to be made up somehow. The bottom of each standoff of the optic has a smaller diameter where it goes through the holes on the mcpcb. The height above the mcpcb is what you need not the overall height. To get a rough idea you could pull out the reflector and hold it up next to the optic and see if there is a noticeable difference.
