Add a custom report to the Memberships > Reports dashboard that displays a report of your membership site’s upgrades/downgrades.
See this blog post on custom reports to learn how to add the custom code to your site.
Create Custom Reports for the Memberships > Reports Admin
Here’s a demo that shows how to add a custom report to the Memberships > Reports page in the WordPress Dashboard.
I’m working on a custom report to show “upgrades” which for our site is members who registered at Free level and “upgraded” to PMPro Member.