Regression testing refers to testing of existing functionality to ensure that recent changes do not adversely affect existing features.
For example, when implementing Asset Management, the existing Incident functionality is tested to test use cases such as creating an incident, creating a task, updating incidents, closing tasks, incidents, etc
In addition to UAT Test Scripts, you will want to do a comparison of existing workspaces, existing functionality, and existing features, to ensure existing functionality has not been impacted.
Ivanti Best Practice: If you want to run Ivanti Service Manager UAT and PROD tenants side by side, you need to use a different browsers (not just different tab), as otherwise the browser cache may become corrupted or cause issues. So for example you can use Google Chrome for UAT and Internet Explorer for PROD.