The steps below apply to Sales Reps incorrectly assigned to either a set of customers or a set of invoices, and can be done in either the Customer listing or Invoice Entry screens respectively.
One thing you don't want to use this for is to change all the Invoices in the system from a previous sales rep to the new sales rep for a set of accounts. While it certainly has the power to do this, what you lose is the historical accurateness of who actually was responsible for a sales, or paid a commission at a given time in history. This type of information is very important from a financial accounting perspective. Beside, the apparent gain in reporting is non-existent since Data Analysis gives the User the ability to see the sales from both the Sales Rep on Customer as well as the Sales Rep on Invoice perspectives; and Report Explorer reports allow the newly assigned sales rep to see all of the sales history.