The greatest concern for an IS auditor reviewing data conversion and migration during the implementation of a new application system is unauthorized data modifications occurred during conversion. Unauthorized data modifications are changes or alterations to data that are not authorized, intended, or expected, such as due to errors, fraud, or sabotage. Unauthorized data modifications occurred during conversion can compromise the accuracy, completeness, andintegrity of the data being converted and migrated to the new application system, and may result in data loss, corruption, or inconsistency. The other options are not as concerning as unauthorized data modifications occurred during conversion in reviewing data conversion and migration during the implementation of a new application system, as they do not affect the accuracy, completeness, or integrity of the data being converted and migrated. Data conversion was performed using manual processes is a possible factor that may increase the risk or complexity of data conversion and migration, but it does not necessarily imply that unauthorized data modifications occurred during conversion. Backups of the old system and data are not available online is a possible factor that may affect the availability or accessibility of the old system and data for backup or recovery purposes, but it does not imply that unauthorized data modifications occurred during conversion. The change management process was not formally documented is a possible factor that may affect the quality or consistency of the change management process for implementing the new application system, but it does not imply that unauthorized data modifications occurred during conversion. References: CISA Review Manual (Digital Version), Chapter 3, Section 3.3