I have never figured out why the first thing a person wants to do when they buy a new orchid is to repot it! The only reason I can think of would be the way the plant's roots seem to grow out of the pot.    I am sure that many of you are not use to having roots growing everywhere but in the pot, but for orchids, this is a typical thing for them to do.

   What you need to understand is most orchids are epiphytes, which are plants that naturally grow attached to other objects (usually trees). It is in their nature to send roots everywhere looking of more points to anchor themselves. Therefore, due to their nature, you will never be able to keep all of the roots inside a pot.
  Also the roots are alive providing nourishment to the plant. Do not cut off roots growing outside of the pot! If you happen to have root rot these may be the only roots the plant has. Some people try to bury the roots that keep growing out side of the pot. This is not a good idea to do.

  There are two main types of roots, aerial and underground. All the roots consists of a central wiry thread which is surrounded by a moisture retaining part which in turn is coated by the white papery covering, called velamen. If you bury aerial roots they will suffocate and rot, if you expose underground roots to the air they will dry out and die.
   That said, there really are times when orchids do need to be repotted. The main reasons you would want to repot your orchid include:

   The best time to repot your orchid is after it has completed blooming and begun to produce new growth. The new roots should be about 1/2" long. Unless your orchid is sick, I recommend that you do not repot an orchid while it is in bloom.

   First assemble what you need to repot your orchid.
This includes the potting medium (soaked if needed), peanuts, pots, something to sterilize your cutting tools with such as a small torch, bleach water and some sulfur to dust the areas that you cut.
  You should also have some small stakes and ties to anchor the plant in the pot. Of course, don't forget new labels for your plants so they don't become no-names. Water your orchid before potting. This helps to loosen it in the pot and makes the roots more pliable causing less damage to them during handling.

   Now that you are ready, tip the pot on its side and gently pull the plant out of the pot. Hopefully, the plant will come right out of the pot.
   If not, you may have to tap the bottom and sides of the pot to help loosen a stubborn plant or you may even have to break the pot to free the orchid. Once out of the pot, if your orchid has been planted in a natural medium such as bark, try to remove all of the old medium from around the roots.
   The best way to remove the medium is to use your fingers to gently pull the roots apart to loosen the root ball. If the potting medium is rocks or a man-made material, you will still want to loosen up the root ball if it is very tight.
   Some orchids will have a massive root ball (for example Cattleya) and others will have many roots but not entwined together (Paphiopedilum). Trim off all roots that are black, dark brown, mushy, or that look like a piece of thread. Healthy roots will be white or light tan-brown.

   Trim your plant by removing any old growth such as "back bulbs," which are older pseudobulb that have lost their leaves but are still alive. Removed back bulbs can be either thrown away or potted in a another pot. Hopefully, they will sprout new growth. Continue trimming by removing any old dead pseudobulb (shriveled and leafless) and throw away.

   Dust all areas that you have cut with sulfur. Now is the time you need to decide if you want to grow a beautiful specimen plant or if you want to increase your collection or trade with your friends. Do not divide your plant if you want to grow a large beautiful specimen plant.

   To divide sympodial ("shared- or common-footed" in Latin, orchids are the type which have a creeping rhizome), look for joints where the plant has grown in different directions, or find an area in the rhizome where you can easily cut the plant. Each cut section should have at least three good growths with leaves.
   Dust the cut ends with sulfur. Place the cut ends to the side of the pot so the new growth will have plenty of room to grow forward.
   Brassavola, Cymbidium, Cattleya and Dendrobiums just to name a few are all sympodial growing orchids.

   To divide monopodials ("single footed" in Latin, i.e. vertical growth with a single apical bud only), you must have a plant large enough that there are roots growing along the stem.
   Cut off the top just below where some good healthy aerial roots are growing. If there are new growths at the base or sides of the old stem, you may remove and pot them. Discard the bottom of the original plant or you can try repotting it in the hope that it will produce new growth (it may not). Monopodial orchids include Vandas, Phalaenopsis and Ascocentrum just to name a few common orchids.

   Terrestrial orchids are divided much like a house plant. You can generally just pull these apart to separate the growths. Ludisia and Paphiopedilum are terrestrial orchids.

   After trimming, you should now have a clean orchid that is ready to be placed in a pot or mounted. Place the Styrofoam peanuts in the bottom of the pot along with some of your selected medium. Place the plant in the pot.
   Position the plant close to the side for sympodial-growing orchids and right in the middle of the pot for monopodials and terrestrials. Fill the pot with your medium.
   If the plant is wobbly, anchor the plant with a stake to keep it firm in the pot.
And do not forget to place the new label with the orchid's name on it in the pot!.