Although older bonsai trees can go several years without being repotted, Young bonsai trees need / should to have their soil replaced every couple years. Not only does your bonsai depend on you to provide it with nourishment and water, it can only glean the materials it needs from soil that has not been depleted. This is why it is so important to take repotting your bonsai seriously. (Repotting Tips & Tricks)
Most Bonsai trees prefer to be repotted in early spring, before bud development begins. In order to start, you will want to ensure that you have everything you need handy, from fresh bonsai soil, to all the necessary tools. Before you begin, be sure to gather your tools in one spot, then, get your tree and put it on your work table. You will want to work quickly, so be sure that everything you need is within reaching distance!
Tip the plant lightly out of the pot, and gently squeeze the soil all the way around until the roots are almost completely exposed. If the tree is root-bound, you will want to use a clean knife to cut the roots free.
Once you have the exposed root system in front of you, you will want to use a hook, chopsticks or a fork to loosen the old soil, and the root ball. You can compost the old soil to ensure less waste. Remove all the roots that are growing directly downwards which will help to promote fresh new root growth.
Be sure to take into account the type of tree you are repotting, and be sure to only take off the amount of root growth that your particular bonsai is tolerant of. Some bonsais are very tolerant of root trimming, and others are less tolerant. However much you trim, be sure that the scissors you use are clean!
Take the bonsai pot, you can change to a new one, or you can keep the old one if desired. Place a thin layer of course drainage material, such as gravel, along the bottom of the container. Place the freshly root trimmed plant into a bucket of water to saturate the roots, sprinkle root hormone on the roots, and then place the tree back into its container.
Fill about half the container with loam granules which can hold large amounts of moisture while preventing the tree from getting root rot through actually standing in water. Fill the remainder of the container with soil. A good mixture for bonsais can consist of equal parts of peat, sand and loam.
Use a chopstick to carefully work the soil in between the roots of your bonsai. At this point, you will wish to test the depth of the pot because the tree should rest so that the base is even with the top of the pot.
If there is any 'gap' then fill under, and around that with soil and pat it down well. Trim and wire branches as desired, and be sure to avoid direct sun and feeding your repotted bonsai for two to six weeks to avoid burning the roots.
Water your newly repotted bonsai well, and enjoy the abundant growth that is sure to prove your skill as a bonsai artist!