If the roots have begun circling around themselves, you need to re-pot. Eventually the roots will grow thick enough to displace all of the soil within the root system, and the tree will starve.
[header] How to repot bonsai trees [title] Determine when your bonsai needs to be re-potted. [step] The primary reason for re-potting a bonsai tree is when its root system begins to choke itself off. To determine whether or not this is happening, gently lift the entire tree from its pot.