If you really want to get the head apart, you need to apply some heat to it- a heat gun or small torch will work. But you have to be very careful as to how much heat you apply- if you get it too hot you can very easily damage the optics, as they are polymer/plastic. You want to get it very warm, not hot.
I took a few of mine apart- E2DL, L1, KL1. I routed out a couple of pieces of wood to hold the head, wrapped it with some thin rubber, and clamped it in a vice. I used heat and cold water alternately to break the threads loose, and a rubber strap wrench to get it apart.
The KL1 head came apart very easily, the other two were a bit more difficult. I took them apart mainly b/c I noticed some dust/debris/fingerprints inside them when the lights were on, esp. on the KL1. Nice and clean inside now!
As for voiding the warranty, I have been told from a few reliable sources that SureFire does not repair or inspect in any depth lights that come back for warranty. They just test to see if they work or not, then will send you a new light. It is not cost effective for them to troubleshoot and repair them. The old ones are scrapped out or sent to R&D for test hosts. If you did have to send it back you could probably just re-lock the threads and they most likely would not even notice it. Do not quote/trust me on this though, as I am only going by what I have heard.
Hope this helps, and good luck.
