This step ensures that any new user in your organization is always created as a Teams Only user.
To change the tenant mode to Teams Only run the following command from a Teams PowerShell window.
Grant-CsTeamsUpgradePolicy -PolicyName UpgradeToTeams -Global
If you don't need to use Skype for Business anymore, you could try to disable the hybrid configuration to complete migration to Teams Only. In this way, the mode will be set to Teams only by default.
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