if it ain't broke, don't fix it.
I wouldn't try it on a pricey Fenix.
that said, I have a UltraFire 3W LED assembly that I would like to mod this way.
I tried it with a rechargeable 3.7V LIon and it works, just need to find the right housing.
that's what I get for buying a fenix copy and not the real thing. the threads were stripped
and wouldn't hold anymore.