One project I started a while back was to fit an H4 in place of the existing H1. I've not got round to completing it yet, but the difficult bits are done.
Enlarging the hole in the reflector to accommodate the bigger bulb base is a slow job, the weird high temperature plastic it's made from is loaded with silica or something that destroys drill bits and hacksaw blades in seconds. Having got a hole into which the bulb will fit, you then need to fabricate a back plate to provide a clamp for the bulb. All a bit of a faff, but the light output is a huge improvement over the original set-up and dipped beam looks almost as well controlled.
Having said that, most people just seem to opt for various aftermarket HID systems despite the fact that they ought to fail the new MOT test.
