A WishList Member License Upgrade can be purchased in the Customer Center.
The cost would be the difference in price between the two licenses.
Example: If you bought a $147 1-Site License and wanted to upgrade to the $297 5-Site License, you would be able to pay $150 to upgrade to the 5-Site License.
Your existing License Key could then be used to activate WishList Member on up to 5 URLs.
The same logic would be applied to upgrading from the 1-Site License to the 20-Site License. Or, the 5-Site License to the 20-Site License, etc.
Any existing Subscription Plan for Updates and Support would remain the same.
All the details and options are available on the Upgrade page in the Customer Center.
You can use the link below to get more info:
https://customers.wishlistproducts.com/license-upgrade/ (must be logged in to access).