While you certainly can just slap some HID bulbs in the reflector housings, you'll get far more usable light by properly retrofitting projectors. You'll also avoid blinding other drivers, which is inevitable when putting HID bulbs in reflectors (the only people that will tell you otherwise are those that already installed them and don't really give a s**t about other drivers). I am blinded almost daily by these idiots now that HID bulbs have become so popular.
In the three and a half years I drove my 05 with the HIDs installed in stock headlights, I had probably less than 10 people flash their lights at me. An HID is no different than a standard halogen in terms of beam pattern, just brighter. It is all about proper aiming of your headlights. If you have them up too high you will blind other drivers, it doesn't matter what the source of the light is it is how you aim them.