RBAC model is a solution which defines users with different roles, and the roles are in different classes which mean the users with different roles have different permission. Usually, we can use secret keys to discriminate the different roles. However, the role in this access control model is a security class including some users. Changes will appear in this system with personnel changes frequently. Due to the keys are corresponding to the roles, how to update the keys in these frequent changes is the focus of this study. There are three kinds of model in RBAC, the linear model, the tree model, and Directed Acyclic Graph (DAG). This paper discusses the changes of users and security class from the linear and tree model. The problem in the method where the inferior keys are determined by the superior keys is also discussed. Thus, key management for changeable RBAC system is effectively realized.