The leds have a voltage bias typicaly between 2.9-3.6v. The will still work fine with a 3v battery. The batteries are in parallel not in series like most flashlights. Eg. Two batteries will still provide only 3v.
Ahh I see Just waiting for my spartan lite to arrive then I can begin tinkering Is the original led round topped or flat top? I've managed to find and 18000mcd cool white leds so thinking I might try one of them.
Are there any other components in there, eg resistors? Or is it just 2 batteries, a switch and an led?
Just the batteries/switch/led and simple conductors.
If you wanted to be creative, you could move the contacts to bypass the secondary batter place(the one the switch doesn't contact to). That way you have a backup battery on board.
All that said, why not just carry a real flashlight and have the dim Lite as an emergency backup?