In the fall of 1992 at the SNEAP (Symposium of North Eastern Accelerator Personnel) a project to upgrade the ATLAS (Argonne Tandem Linear Accelerator System) control system was first reported (1). Not unlike the accelerator it services the control system will continue to evolve. However, the first of this year has marked the completion of this most recent upgrade project. Since the control system upgrade took place during a period when ATLAS was operating at a record number of hours, special techniques were necessary to enable the development of the new control system “on line” while still serving the needs of normal operations. This paper reviews the techniques used for upgrading the ATLAS control system while the system was in use. In addition a summary of the upgrade project and final configuration, as well as some of the features of the new control system is provided. ©1999 American Institute of Physics.