When upgrading from Teams Free, I contacted Microsoft Support and confirmed that the following public information steps can only be completed with a direct contract that allows you to change the number of licenses depending on the number of users in your tenant.

https://support.microsoft.com/en-us/office/29475bbd-a34f-4175-9b33-d44430f8ad39

In the end, I couldn't upgrade using the public information procedure above, but in operation I completed domain ownership verification and was a tenant administrator, so I signed a contract with a promoted management tenant. .. By assigning a license to the user, it started to be used Teams as a paid version. Become an administrator for unmanaged tenants created by self-signing up to Microsoft 365 services As public information is not provided, it is difficult to reach the goal, so a wizard to become an administrator Please provide or provide a CSP contract or a wizard to complete the prepaid upgrade.