Question about maintaining mounted orchid

[gngrhill]

I recently received my first mounted orchid, (thank you, Judith) . The roots have Old SM under them and new SM over the top of them. Plant is doing very well, but I wonder if the old moss underneath will need to be changed and if so, how does one accomplish that feat ? I know in potting, media has to be renewed every year or two. I don't know how you would do that with a mounted one. It is on a cedar slab. All advice on the matter will be welcome. thanks