Potted orchid plants, be they terrestrial or epiphytic, should be transplanted into fresh growing medium once a year after flowering. The organic compounds in orchid bark break down as it ages and over time it loses its capacity to hold moisture and nutrients to the plant roots. Increasing the size of the pot every three years or so will keep the orchid from becoming root bound but still provide the light root compression that encourages blooming.
Water the orchid well before attempting to transplant it by running the plant in its pot under tepid water from a gently flowing faucet. Wet roots are easier to work with as they are more flexible and it is easier to remove any old potting medium from flexible roots. Run the water for a few minutes and allow the excess water to drain away.
Slide the orchid plant out of its pot while tilting it on it side. Pull the plant gently to free it from the pot and hold the plant with its roots resting in and supported by your palm. Carefully brush away the old potting medium that surrounds and is stuck in and amongst the roots. Set the orchid aside for a moment.
Fill the orchid pot with an inch or two of fresh potting medium suited to your varietal of orchid. Use very coarse, lightweight and fast draining medium with no soil for epiphytic orchids such as dendrobiums. Choose a finer grain bark mixture with some soil for terrestrial orchids such as paphiopedalums.
Place the plant into the pot and gently distribute the roots evenly throughout the plant. Ensure that the crown of the stem sits one inch below the lip of the pot and add medium under the roots if needed to lift the plant to this level. Fill in the new medium around the roots carefully using your fingers and a pencil or thin dowel to gently push the medium between the roots so that there is good contact between the roots and planting medium.
Water the orchid and fresh planting medium well under tepid running water until it is soaked. Let the water drain through for several minutes and place the orchid back in its usual growing location where it receives bright indirect light and resume your regular watering and fertilizing schedule.