(Right) The roots on this phalaenopsis orchid are extremely healthy. 
The plant is growing in sphagnum moss in a transparent pot. The roots tend to grow towards the light and are also slighlty photosynthetic. Transparent pots make it easer to asssess how the roots are doing.

When to Repot Your Phalaenopsis Orchid
Repot plants about every 12-18 months, or when the mix decomposes. Bark mixes will turn dark brown and crumbly, often staying quite wet.

Try to repot your phalaenopsis orchid before they get to this stage. Use a mix of 2/3 medium fir bark with 1/3 coarse perlite, or a mix especially formulated for phals.

How to Repot
Gently tug the plant free of its pot while wobbling it from side to side. Take care not to damage any living roots. Carefully remove any dead mushy roots, again trying not to damage any roots that are firm and white.  Also be careful not to break the living roots when you repot. Remember, an orchid is only as good as its roots and these tend to grow slowly.  Shake as much of the old fir bark of the roots as you can manage. Sometimes a stake or chopstick helps in dislodging mix from the center of the root ball. Allow dead dried roots to break off and remove any that are clearly dry and stringy.

Repot the plant in a slightly larger pot if needed. Remember that phalaenoipis orchids like to be a little pot-bound. Hold the plant in the centre of the pot, with the lowest leaf about 1/2 inch above the top edge of the pot. Fill in the pot with bark mix until the plant is held in place. Then using a stake, gently work the potting mix in between the roots. Add more mix as needed and finish off by tamping the mix firmly among the surface roots iwth your thumb. Do not try to force any rots growing out of the pot in the mix. Your phal should be sit firmly in the pot and not wobble. Soak your plant thoroughly. Keep it more shaded than usual for a few weeks while they adjust.  Healthy roots tend to be  firm and white with green growing tips.

Peat-Based Mixes
Phalenopsis orchids do very well in certain peat-based mixes such as Pro-Mix HP or certain types of Fafard mizes available from specialty orchid supply stores online.  Do NOT use regular houseplant soil, even if is peat-based!  Pro-Mix lasts longer than bark. When you do repot, its very easy to simply shake the mix free of the roots. Follow the instructions above, except do NOT compress the mix into the pot and around the roots whne you place the orchid plant in its new pot. Tap the pot gently, topping up the mix as it settles in between the roots and the plant is firm. Water the plant thoroughly. Do this a few times over the next day or two, as the mix will initially be resistant to absorbing water.