The primary focus of the IS auditor reviewing the first year of the project should be regression testing.
Regression testing is a type of testing that ensures that the existing functionality of the system is not affected by the changes or upgrades made to the system. Since the project involves upgrading the ERP system hosting the general ledger, which is a critical and complex component of the finance department, it is important to verify that the upgrade does not introduce any errors or defects that could compromise the accuracy, completeness, and reliability of the financial data and reports. Regression testing can help identify and resolve any issues before they affect the users and the business processes.
Unit testing, network performance, and user acceptance testing (UAT) are also important aspects of the project, but they are not the primary focus of the IS auditor in the first year. Unit testing is a type of testing that verifies that each individual module or component of the system works as expected. Network performance is a measure of how well the system can communicate and exchange data with other systems and devices over a network. User acceptance testing (UAT) is a type of testing that validates that the system meets the user requirements and expectations. These aspects are more relevant in later stages of the project, when the system is more developed and ready for deployment.
References:
ERP Upgrade: The Path to Modernization | SAP
ERP System Validation: Your Guide To Successfully Validating ERP Systems The role of internal auditors in ERP#based organizations What is Regression Testing? Definition, Tools and Examples What is Unit Testing? Definition, Tools and Examples What is Network Performance? Definition, Metrics and Examples What is User Acceptance Testing (UAT)? Definition, Process and Examples

Access to production source and object programs is the best way to ensure that effective change management is in place in an IS environment, as it prevents unauthorized or accidental changes to the production code that could affect the functionality, performance, or security of the system. Access to production source and object programs should be restricted to authorized personnel only, and any changes should follow a formal change management process that includes documentation, approval, testing, and review.
References
ISACA CISA Review Manual, 27th Edition, page 254
Change Management Best Practices for the Engineering and ...
Change Management - an overview | ScienceDirect Topics