Many enterprises have implemented packaged enterprise resource planning (ERP) solutions rather than system development on the promises of feasible strategic benefits. Typical feasible benefits include adoption of global standards and processes through a proven integrated system, continuous version upgrading by vendors, and thus the easiness of system maintenance. The objectives of this research are to identify what makes a post-implementation success of ERP system and to figure out the critical issues and factors for the successful upgrade of packaged ERP systems. This case study was conducted based on five different organizations that have experienced system upgrade implementation. Success factors of ERP system upgrade include (1) clear statement of upgrade goals, (2) good coordination and communication among the implementation partners, (3) thorough management of customization and maintenance history including documentation, (4) Planning of system requirements of higher version, and (5) redefinition of role between business departments and IT department.