Hello @Simon Burbery ,
Here are the notes for upgrading Windows 10 Pro to Enterprise for your setup
Devices must be running Windows 10 Pro, version 1703, and be Azure Active Directory joined, or hybrid domain joined with Azure AD Connect. Customers who are federated with Azure Active Directory are also eligible.
There are many methods available to assign license. I haven one of them
Sign in to portal.office.com and open the user properties as shown below and assign the license:
Now that your Windows 10 Enterprise E3 or E5 licenses have been assigned to users, the users are ready to upgrade their devices running Windows 10 Pro
To join a device to Azure AD when the device already has Windows 10 Pro, version 1703 installed and set up
- Go to Settings > Accounts > Access work or school
- In Set up a work or school account, click Join this device to Azure Active Directory under Alternative Actions
- On the Let’s get you signed in page, enter the Azure AD credentials, and then click Sign in
- Now the device is Azure AD joined to the company’s subscription.
- Verify that the license is activated
Please mark as "Accept the answer" if the above steps helps you. Others with similar issues can also follow the solution as per your suggestion