Surefire makes durable and bright flashlights for the military. The bulbs seem last only a few hours and and genuine replacements cost $20-$30. eBay lists direct-from-China (search for “Cree Q5 led bulb 1-Mode For Surefire”) replacement bulb assemblies for around $6 and an LED version for $9 including shipping.
The replacement bulb simply screws in but the length is slightly longer leaving a small gap slightly compromising the water-tight seal.
The LED brightness seems to be similar to factory P60 incandescent bulb. The light distribution is different as is color having a blue hue.
One would expect the LED bulb to have greater longevity and battery life over the incandescent but this was not tested.