Carrie, welcome to BOL.
A loan whether open end (creditline) or closed end (installment, mortgage loan, etc) is a legal agreement between the bank and the customer. One cannot change it without the concent of all parties. In some cases, there are changes that can be triggered by default collateral, and had already been agreed to.
Legal is necessary like Randy and John mentioned.
Integrity. With it, nothing else matters. Without it, nothing else matters.