Change function can be used as a mathematical term. In calculus the change function is found when looking at the derivative. The derivative is a measure of how function changes. In business change function is not always going to be mathematical.
Instead change function is a concept regarding how a business needs to change and what function it needs to change under. Generally change function looks at the business, how it has succeeded, and what can be different to gain more success. Over time a business has to change. Without a change in function the business will lose the consumer interest. In fact many companies have not succeeded because they have not updated their function to go with the new technology. Businesses are always looking for solutions to problems, or they should be.