How to get a notification when a workspace admin changes a workspace from pro to premium capacity?

Notifications no, but you can see the details after the fact (with a 24 hr delay) in the Power BI implementation planning: Tenant-level auditing - Power BI | Microsoft Learn tenant audit logs.

No, you run daily scheduled powershell scripts with a real scheduler, and then store the received JSON in your audit log database of choice. Power Automate cannot handle Powershell scripts, and it cannot handle the file sizes.  You need real tools for that.