Assuming you are privately held, there is no requirement to change audit firms. In fact, you may find it cost prohibitive jumping firms every few years. Sarbanes-Oxley recommends a review and a change in the audit partner every 10 years (I believe, can't check since I am at home). We require a change in audit partners and the audit manager every 5 years.
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Societies that do not find work in and of itself "pleasing to God and requisite to Man," tend to be highly corrupt.