It's actually a relatively easy mod to do yourself. I've done one of mine with an XP-G and recently put an XM-L in another. Here's a fairly comprehensive thread with instructions and photos:
The XM-L in particular is really an eye-opener - very bright, with a much beter tint and a wider, very useful beam. The XP-G provides a narrower beam which throws better and is closer to the original, although still not quite as tight.
