I believe there is a difference in the wiring in that when you flip from dip to main beam on a normal headlight bulb, one filament goes off and one comes on.
I think on the 03MY headlights, when you hit main beam the dipped beam stays on as well, in which case you'd need to twiddle with the wiring to account for that (or put up with less light output).
I may be wrong though, but I think I saw that somewhere. Don't have an 03MY to look at at the moment, mine has got proper Discovery headlights in it not horrid four eyed ones