Moding lights is surprisingly complex. It's not just a simple remove and replace. Go over to candlepowerforums.com and check out the threads there.
By no means an exhaustive list:
How many lumens the emitter produces is a function of the current it is given.
The voltage from the battery will drop continuously if not using a driver (see dealextreme.com for many of them)
the driver needs to match the battery pack and the led(s)
don't wire together a LiIon battery pack without knowing what you are doing unless you want to make an explosive device
By no means an exhaustive list:
How many lumens the emitter produces is a function of the current it is given.
The voltage from the battery will drop continuously if not using a driver (see dealextreme.com for many of them)
the driver needs to match the battery pack and the led(s)
don't wire together a LiIon battery pack without knowing what you are doing unless you want to make an explosive device