Upgrading Block method fees to Function method fee needs to be done manually.
From your Shopify admin, go to Online Store > Apps.
Select Magical Product Fees.
In the app, open Rules from the navigation menu.
Click on the fee you want to upgrade.
Copy all the settings, set the fee to Draft and click Save.
Back on the Rules page click Create fee and select the Function Method.
Update the settings to what you had copied for the Block Method fee.
Click Save.