License downgrades should be able to be performed from the VMware Customer Connect portal. Go there and choose the Manage Licenses icon from the main Customer Connect page.

Fusion 13 should be viewable in the products list if you've purchased/registered a license for it (it'll be there automatically if you've purchased a license directly from VMware, otherwise you'll have to register your license). The default is to list all of your products. Click on the "Downgradable" button to view products that can be downgraded, and Fusion 13 should still be visible.

Click on the Fusion 13 product. The license key(s) associated with any purchases of Fusion 13 should be shown. Check the box next to the Fusion 13 license, then select "Downgrade" as one of the 2 actions.