I changed the lens to glass, upgraded the emitter from s bin to a t bin lux3 and then to a q5 cree. Sevral years ago i found a thread here on upgrading the heatsink ring. What I did was sand down a few pennies and then soldered them together and press fitted the copper slug into the ring. I wouldn't sugesst you bother with this one. It's way too much work and isn't nessesary. Never changed the driver board. You'd probably have a hard time finding a driver board as big as the one in the q3 to do an easy upgrade. I think you would have to strip the components off of the original board and then rig a smaller driver to it.
It was a nice light in it's time but now I feel it's too big for a single 123 light. Get more lights is my advice. You can't have too many.
If you're really bent on upgrading it, do an emitter upgrade and the glass lens.
i put an edixeon star in mine with ims 20 reflector and glass lens. i run it on a rechargeable with excellent results. i measure it in my light box the same total output as my d-mini, (also on a rechargeable)
I did have to modify the reflector a bit for my warm cree. A tiny bit of dremel work and it was perfect.
Kaidomain supplied my parts (so far were square (though I had to use paypal's dispute resolution on them a number of times) they have stuff I can't find elsewhere: Forward clicky coated glass lens Warm cree on a star
This bit of surgury seems more trouble that it's worth! There is this really nice Fenix I was looking at but as my old P1 died, I've had to back off from another Fenix till I can find what why. And it doesn't seem as it the folks at Inova are going to create a single cell unit anytime soon. Thanks fellas, now it's time to go shopping!