All orchids should routinely be repotted every 2-3 years. They enjoy a well-drained mix such as orchid bark. After 2-3 years of the plant being in the same pot, the bark/mix degrades and does not provide adequate drainage.
The best time to repot orchids is in the spring when the plant shows signs of new growth. The least favoured time is while the plant is in flower; however, sometimes it is unavoidable. You may choose to divide your plant at this time. Divisions should be no smaller than 3-5 bulbs or canes. This enables the plant the best chances of reflowering during the next season.
The size of the plant or division dictates the size of the pot you use. For most orchids mentioned here, the pot should be no bigger than 3-5cm bigger than the plant itself. This gives the plant enough room to grow for the next 2-3 years.
The one exception is phalaenopsis. Select the smallest sized pot that will fit all the roots. Sometimes it is necessary to pot your phalaenopsis into a larger pot, usually because the plant has become top-heavy. Generally, you will keep to the same size pot.
When repotting your orchid, it is a good time to check the roots. Trim off any dead roots. You can also see if you are under- watering or over-watering your plant. If the new growth is tending to go towards one side of the plant, place the non-growth side of the plant hard up against the edge of the pot to allow more room for the new growth to expand into, otherwise, keep the plant centred in the middle of the pot.
Keep the plant evenly damp and well fed while it re-establishes itself in the new mix.