If you have a 503CW, the screen in it should already be an Acute matte D, I think. So, unless somebody has swapped it around, a new screen won't be brighter, although you should be able to get a different version without the split-image rangefinder (I really like the split image myself).

I would recommend contacting the ProCentre in London for info on getting the screen you want - they still deal direct with Hasselblad I think, and I always found them pretty helpful.

Ian