That sounds you're preparing for war with your customers from the outset. Don't go there! Expect that they'll change their mind about things, and be prepared to adapt. Part of being prepared is being light on your feet with documentation, to minimize that amount you'll need to change when new requirements come in and you have to make changes.

If you're concerned about schedule dissatisfaction, prepare a short document for each change request that recaps the changed requirements, and details the consequences (in terms of schedule and resources) of the changes. Get the customer to sign these.

Or better, engage your customer more frequently, and have them help prioritize the list of what they want to see next. If you're following one of the Agile methodologies (e.g., XP), you'll be in a position to "release" fairly frequently. Giving the customer the chance to reprioritize lets them change their mind, and greatly reduces the chance of them coming back at you with a "this isn't what we wanted!" argument.