We have to repot for various reasons, but if you are here, chances are you are repotting a fairly new plant, the leaves are floppy and so you think that you may have over watered and have root rot.
Don't worry, your plant is remarkably tough and resilient. Phalaenopsis orchids often seem to put on a growth spurt after being repotted. The thick leaves are much like those of a succulent. They store enough energy for the plant to grow a new set of roots, so if a few roots are damaged during repotting, it is no big deal. If you do have a large amount of root damage, the lower leaves of your plant may shrivel, and may even turn brown, dry up and die as the plant uses the stored energy and water to get restablished, don't worry about it, just be aware that the plant may do this and it will be stronger for it. If you have major root rot, but you still have one or two roots that are a few inches long, I suggest simply repotting into a slightly smaller pot with your usual bark mix, and then takeing extra care of the plant until the new roots can get established. By doing this instead of "Sphag and Bag" your plant will be seated well in the pot and the new roots can grow directly into the mix and become established more quickly.
Even if you do have a lot of good roots, your plant will have a setback for a few weeks (but it may be so slight that you will not notice it). This is because in repotting you are breaking all of the absorptive contacts between the old peices of bark and the roots so the newly repotted roots are not able to take up water as efficiently as they did befor. Watering more frequently will do more harm than good because it will adversely affect the potting medium, and may encourage undesirable microbial growth.
Befor you start repotting, it is necessary that you soak your bark mix for at least 24 hours to fully hydrate it. If you pot in dry bark, the bark will not want to absorb and hold water. If you ahve already potted in dry bark, submerge the pot in a container of water and soak it for several hours with the plant in it, and repeat this every time you water for the next few weeks.
To get the plant ready for repotting, I let the pot get dry, and on the day you would normally water it, do just that, water and fertilize, and let the plant sit in the fertilizer for about 2-3 hours to soak up as much as it can. This also lets the roots become more pliable so they will come away from a clay pot, and it will also cause the bark soften so it too will come away from the roots. This will result in less damage to the root velamen, and reduce the chances of cracking a root.
Unpot the plant, and get all of the old bark off the roots. Run it under the tub or sink faucet to help. Then set the bare root plant on a towel to get the liquid water off, take a scissors or sharp tool of choice and snip off the dead roots and any that get in the way of positioning the plant well in the new pot. Dump a little pile of cinnamon (a natural bacteriacide and fungicide) onto a plate and dab it onto the cut ends (I use a butter knife to get a little pile from the big pile and dab it on the roots). Let the plant then sit over night on the towel to dry and heal the cuts.
The next day put the plant into the new pot so that the lowest leaf touches the top of the pot. If you have some long roots that will not let your plant be centered or placed as deep as you want go ahead and cut them shorter. Work the bark mix into the roots. I use a little extra loose horticultural charcoal to pour into tighter places. Don't feel bad if you have to dump the plant out and start over a couple of times. I am never happy the first time. The plant should be centered and up right with the bottom of the first leaf base touching or even down in the bark slightly.
Put the plant in a shady but not dark place undisturbed for at least two weeks. You can put a plastic grocery bag LOOSLEY over it if it starts to get very wrinkly, but you should not need to do this simply for floppy leaves if you have good roots. A plastic pot may not dry out before two weeks under comfortable home conditions. Then only water the plant, and move it to a slightly brighter area… no fertilizer for another week. Start fertilizing again and move the plant to higher light.
In order to tell if your plant needs water you can take a bamboo skewer and leave it in the bark, pull it out and if it is wet (cool when touched to your cheek) your plant doesn't need water yet. After a while you get a feeling for a light pot. The plant itself will be much heavier than the pot and want to fall over. All of us have a tendency to want to water and fertilize so we tell ourselves that the pot is lighter than it really is. It is much better if we get on a schedule (I water on Saturday but only if the plants needs it, because in the winter or if it has been raining, the plant can go two or more weeks. A healthy plant can also sit out bare root for a couple of days with out any problems, so it can easily make it a week). A 4 inch pot of bark will barely dry out in a week, larger ones take longer.
Note at End
For repotting other types of orchids (those having pseudobulbs) it is common practice to position the plant so that the oldest pseudobulb is against the side of the pot and the newest pseudobulb is placed in the center of the pot positioned upright with approximately 1/4 of its base covered by mix. This may mean that many of the older pseudobulbs are nearly or totally buried in the mix. It is not uncommon to see pseudobulbes shrink and wrinkle as the plant usedsenergy to become established. One major nursery suggests that Oncidiums be repotted after letting the plant sit bare root for 4-5 days for the roots to dry. I have had good luck repotting these with moist roots as described above.
For all types of orchids choose a pot that will allow for two years growth. if you are repotting a plant that has root damage, you want to use either the same size pot it was in, or a size smaller. many orchids will not bloom unless slightly root bound. Over potting causes the plant to put its energy into root mass instead of leaf mass or flower production. It also leads to water retention and fast decomposition of the mix.
Another note at end.
Like any one with a new hobby involving a living thing, I too worried about killing my delicate and expensive orchid (hey, $10-$20 is a lot of money for a house plant). After all, the books say how necessary it is to have warm temps and humidity, mist twice a day etc. Now I know different. Nurseries simply tear a plant out of one pot, shake it off, stick it into a larger pot, toss in some bark mix and put the plant back on the shelf in a section of the green house with lower light. Many of us repot a new plant into a mix we are used to using the minute it comes in the door. This way there is no danger of root rot from the plant staying to long in old mix. We therefor often try to buy plants bare root (un potted) from mail order companies to save shipping cost. A few days out of the pot in somewhat humid conditions does not hurt the plant at all.