This may be intersting in General:
https://www.candlepowerforums.com/vb/showthread.php?307358-Surefire-10X-Eneloop-Battery-Replacement!
I do not know exactly how the pack is build up exactly. My guess it that it can be opened in a way somehow. Than you only need to replace the old NiCd cells with cells of the same size in NiMH. The charger should work since NiCd and NiMH cells are having same characteristics.
Can you see that the pack itself has some screws of a sprint that keeps the pack closed? That would be the expectation on how Surefire did this battery pack. Or if there is nothing to see it is also a try on asking Surefire to give an advice on opening the battery pack for cell replacement.
The only this may be getting the correct cell size "A" cells are not often in use anymore, but you should get this cells with NiMH chemistry.