Growing phal in sphagnum moss

[opulenceflowers]

I grow my phal orchids in spagnum moss (which I bought them in). I have read that it has a very long shelf live, but how do i determine if I need to replace my sphagnum moss with new? The Phals are also supplied in a 100mm diam pot. When do I need to repot it to a bigger pot. They have never lifted themselves out like a cymbidium but still produce new leaves? Lastly I noticed in one instance that a view roots on perimter of 1 plant went black and died - is this natural or is is wrot due to overwatering?