Repotting is a crucial step in the cycle of a Bonsai. It is necessary to keep the tree healthy but at the same time a delicate procedure that can ruin it. Repotting is necessary to cut back the roots, to change the soil and allow more oxygen to penetrate the root base. Normally it is being performed every 3 years depending of the tree and its age.
It is a very delicate procedure because you need to take the tree out of its pot and cut back the roots in a quite drastic manner. Also the tree needs to be repotted with new soil and grid. Usually a mixture of Akadama, Bims and Kiryu is used for optimal drainage. Afterwards the tree needs to be tied up in the pot. Once that's being done it needs to be watered well including the foliage.
These many stages during the repotting are crucial to prevent the tree from becoming sick or ultimately dying.