There is no support to convert the evaluation version to a fully licensed version of enterprise edition.
The technical upgrade path from the Windows 10 Enterprise evaluation to paid seat of Windows 10 Enterprise (through VL) is a full OS install.
Please go to VLSC to download enterprise edition or go to official website to download Pro edition for use.
To take advantage of E3, you must have the following:
-Windows 10 Pro, version 1607 (Windows 10 Anniversary Update) or later, installed and activated, on the devices to be upgraded
-Azure Active Directory (Azure AD) available for identity management