I think you are talking about the focussing screen.
I can change my 5D focussing screen myself as it is easily detachable.
But there is on additional step for the 50D that requires screwing.
Thus for convenience sake, I send both to the camera shop to change right on the spot.
About $55 each.
(P.S. I am not sure about the 500D, but I reckon it would be fairly similar.)