New user or user left from organization in ACL

Hi..

I am implementing the SAP DMS in one of my client.I have below case.Please suggest the solution.

1.How i can replace the user by other user from DMS ACL on all documents once he left the organization. (Need Mass change option)

2.How i can add new user on the documents which are already created? (Need Mass change option.Close to 500 number of docs will be going to affect)