Shouldn't think it'd be too hard - the ballast for the hid lights is still present which is the main thing, although iirc they filed down the bulb connector in my car to fit the halogen bulb housing in... but i could be wrong.
I would have returned mine to hid, but the bulbs are £45 each so i didnt fancy risking £90 for it not to work!!
Nixy, you'll know if they are hid - i believe they flicker very slightly as they are turned on, and they have more of a blueish tint to them - not even a hint of yellow.