That's a rather complex answer. It would depend on the functional specifics of the application, compatibility requirements, error trapping expectations and whether it runs stand alone or has external datasources or calls.
1. Functional specifics - does it do what is expected?
2. Non-best path - does it not do what is not expected?
3. Compatibility regression for other supported versions
4. Error trapping - does it fail in a predictable way?
5. Boundary testing data entry fields
6. Access control - can users access that which they should not?
7. VBA unit testing - do functions/subs perform their stated tasks?
8. UAT - is the interface and usability acceptable to the user base?
9. Database interaction - Are SP's/calls returning data formatted as expected?
10. Automated regression scripts for future dev cycles