To assess the impact of GUI changes on existing users:

1. Conduct user surveys: Gather feedback through surveys, interviews, or focus groups.

2. Usability testing: Observe users interacting with the new GUI.

3. A/B testing: Compare user behavior between old and new GUI versions.

4. Analytics analysis: Monitor user behavior, engagement, and performance metrics.

5. User feedback mechanisms: Collect feedback through in-app feedback tools or support channels.

6. Heuristic evaluation: Expert review of the GUI against usability principles.

7. User acceptance testing (UAT): Involve users in testing and validation.

8. Change management: Communicate changes and provide training or support.

9. Monitor support requests: Track changes in support requests or issues.

10. Long-term monitoring: Continuously assess user behavior and feedback after changes.

By using these methods, you can assess the impact of GUI changes on existing users, identify potential issues, and make data-driven decisions to improve the user experience.