Replacing bulbs

Are you talking about a HID (High Intensity Discharge) kit or a bulb which gives a white effect??

If you are talking about a HID kit then yes it is perfectly safe on the proviso that you are installing them into projector headlamps (they have a dome that the light goes in- you can tell by looking at the lights) as they will concentrate the light better, if you have the regular reflector headlamps then it's an absolute no, they will blind other road users as the lights is spread all over the road, could cause a serious accident.

Companies such as pro parts, AC cars (both site sponsors) can sort you out for the right one in your car.