From what I understand, the conversions are as good as the manufacturer's ANR. ANR is great, especially for those of us who have already damaged their hearing in their youth getting stupidly too close to loudspeakers when at loud concerts/clubs. My ears ring after a long flight when I use passive headsets, nowhere near as bad using ANR.
Make sure you don't lose the battery boxes, they are very expensive bits of plastic to replace. I duct tape the box to the cable now so it doesn't get left behind.
While you are at it, I'd also buy gel seals and cloth ear covers. They make longer flights far more comfortable, for me at least.