You may have put your orchids outside for the summer. The plants benefit from the high humidity, and the temperature change between day and night helps to spur their re-blooming cycle.
Now that the evening temperatures are dipping into the 50's, it is time to bring them in until next year. This requires repotting to prevent bringing inside any unwanted guests (insects, bacteria, fungi, etc.)
Since orchids generally do not rely upon the potting material for nutrients, gas exchange, and water absorption, potting soil is not used. Orchids rely upon the potting medium mainly for anchorage and stability.
Orchid bark, sphagnum moss, and coco fiber are most commonly used. Bark is a good all purpose material. It comes in differing grades of coarseness. Finer roots require a finer blend and larger roots will require a coarse mixture. Sphagnum moss works well with orchid types that typically benefit from somewhat moister conditions. Coco fiber is another all purpose material, but it comes in only one type of grade.
Soak the potting material for 2 to 4 hours before using. It is packaged in a very dry state and takes a considerable amount of time to become fully hydrated.
Choose a pot with drainage and enough room to hold the roots without cramping them. It should also allow about 1 or 2 inches along the sides for growth.
Gently remove the orchid from its original pot. Clean the roots of all debris. Examine the roots and trim off any that are shriveled or discolored and rotting. The orchid roots are a good indicator of the plant's watering needs. If the roots are shriveled and dry, the plant needs to be watered more often. Discolored or rotting roots indicate that the plant is receiving too much water or needs better drainage.
Fill the new pot about a quarter of the way. Hold the orchid in place and fill around the roots with the wet potting material. Leave about an inch at the top of the pot to allow for watering. Stake the plant to hold it in place until the roots develop enough to provide stability.
Continue with your normal watering schedule, but do not resume fertilizing until new root growth develops.