There are two main reasons for repotting an orchid. First, you have done a wonderful job of caring for your orchid and it has outgrown its pot. Second, you are entering your orchid in an orchid show; and the show requires a certain type of pot. Regardless of the reason, it is always best to repot an orchid after it is done blooming and before it enters into its active growth phase. This approach does not affect any blooms the orchid may have and allows the new root growth to occur in the new pot.
I’ll address the second reason for repotting first. Some orchid shows have very specific types of pots which must be used when entering an orchid. Many times, you may not realize that you will be entering your orchid into an orchid show until it begins blooming. Both of my first-place ribbons in the Philadelphia Flower Show came from orchids that I had not expected to enter in the show. Once the orchid is blooming, it is best to not have to disturb it by repotting. This is where the technique of “over-potting” comes into play. “Over-potting” is when you place your orchid in a show-acceptable pot that is just slightly larger than the one it’s potted in. Use stones or gravel in the bottom of the bigger pot to add weight and keep the orchid from tipping over. Then, gently stuff sphagnum moss or orchid media around the sides to cover up the smaller pot. For all intents and purposes, the orchid will look like its potted in the larger pot without disturbing the blooms. No matter how careful you are, repotting an orchid that is blooming will have a negative effect on the blooms. I have included an image below that can be snipped and printed, if needed.
Once an orchid has stopped blooming, it can be removed from its pot and repotted in a larger one. Try to pick a new pot that is just larger than the original one. Bigger is not necessarily better. I have found that using a pot that is too big will cause the orchid to spend all its energy producing enough roots to fill the new pot. It does very little growing above the ground or blooming during this time. I have included repotting instructions below that can be snipped and printed for reference. Feel free to contact me via email at firstname.lastname@example.org if you have any additional questions.