Answer: Since the original mix is breaking down, it sounds like you do need to repot. The best time to do it would be when new growth is starting or right after it blooms. Repotting is not too difficult, if you have ever repotted a house plant then it is quite similar to that.
Use an orchid mix similar to the original-- the same is best -- and a pot that is only a little larger, about one size bigger. Put some pot shards or pebbles in the bottom of the pot to assure the roots will never sit in water. The new container must have drainage holes. Soak the new medium, the new container if it is water permeable, and the roots for fifteen minutes or so before repotting. Be gentle with the roots.
Depending how it is situated now, you may want to break or cut away the old pot to avoid disturbing the roots when you take the orchid out. Remove the old medium as best you can, trim away any dead, black roots (use a sharp, sterilized knife; some orchid growers will use a fungicide such as cinnamon on the cuts.)
Reset it into the new pot, work the medium securely down through and around the roots. Make sure the stems are above the medium and the roots are well covered. You may need to stake it gently to keep it stable until it re-roots. If you use a slow release fertilizer you could add that now, too.
If you are still unsure, perhaps a local orchid supplier would walk you through it or demonstrate on one for you. I hope this helps!
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