The Adjustments export is the best option to finding the data you need. Here are the columns you would use to find that data within that export.
1. After downloading the Adjustments export, you will first filter the "adjustment_type" column to only show "credit." This will remove charge invoices from the list.
2. Next, you will filter the "invoice_type" column to only show "immediate_change." This will ensure you only view credit invoices associated with a subscription downgrade.
3. You can use the "adjustment_created_at" to determine the date the downgrade occurred and the "adjustment_amount" column to calculate the total amount of credits created.