Right now OrderMetrics does not offer a feature where you could define a different transaction fee rate for different time periods. For example, if you were to update the rate for Shopify Payments from 2.9% to 2.6% in OrderMetrics, the change would apply retrospectively to past orders as well.
A workaround would be to create a custom spend to counter the effects of a lower rate when applied to historical orders.
To do this, you would first calculate the difference in transaction fees after the lower rate is entered for the same range. For example, let’s say you were seeing a total of $1,186 in transaction fees for all time. When you update the rate, this number should be decreased. Let's say it decreased by $300. Now we'll take this number and create a custom spend with it for the same timeframe. Head over to Data Warehouse > Custom Spends and create a custom spend like so:
Note that you would need to repeat this process every time you change the rates to a lower rate.