I have had a 30" bar for several years now. I also own a 4" bar. They are excellent lights! I do wish I would have bought two 20" bars instead but oh well. If you are contemplating the 40" bar, I am sure you will be happy with it. It shouldn't really need upgrading at all... unless you are super fast. What kind of offroading do you do anyway? That would make a huge difference in my mind.
If you do feel the need to upgrade, they are very simple to take apart. The only snag I see is that the LEDs are mounted directly to the circuit board. My upgrade plan involves using LEDs on stars and replacing the circuit board with a thin strap of aluminum to improve the thermal path. And then of course, there is the hunt for the drivers. But overall, I would say the housing lends itself very well to being used as a host.
The reflector/optics units also will be useable. They seem to be a very efficient design. The four different reflector bases are spaced far enough apart to fit four 20mm stars mounted underneath. Each unit is molded so that the LEDs should be focused pretty well. The hole at the bottom of each reflector is physically large enough for the XM-L dome. I would be afraid that the XM-L would turn it into a monster flood light though. But I would say if your looking for more flood than throw, go for the XMLs!
For reference purposes, my light has P4 LEDs in it. (I actually bought my light when Holder was still producing them - before Rigid purchased the line of lights.) I replaced my reflectors with all spot reflectors for improved distance. It still produces a floody light but it has enough throw to allow me to safely react at about 65 mph offroad I would say. (If I use my stock headlamps, I start outrunning them at the 40-45mph range in the open desert.) With a well defined road the light gives me enough reaction time for higher speeds - hehehe.:naughty: I have personally been thinking that XP-Gs driven at one amp or so would be a better option than XM-Ls. If I ever get myself in gear, I plan to test with the 4" light first.