Edit: 1/11/2019 - Seems that Teams Only mode is starting to appear in the admin center again.
I've been seeing a few posts around the community that they cannot upgrade their Tenant to Teams Only mode through the admin center and at first I believed that it might be just an update that needed to take place on the back end for this ability to show up, but now I believe it's an admin center issue and the best way to set these items is through PowerShell.
At some point about a month ago I was able to set my own upgrade policy to Teams Only in the new Teams admin center UI and even had the option to set it for the tenant, but recently discovered both these options have magically vanished from the UI in the new admin center.
Wanting to set my tenant now to Teams Only mode I figured I would check PowerShell and sure enough you're able to easily do this using the Grant-CsTeamsUpgradePolicy commandlet.
There is a great article touching over all the interoperability operations and settings at https://docs.microsoft.com/en-us/MicrosoftTeams/migration-interop-guidance-for-teams-with-skype
If you want to set a single user set to Teams Only or one of the other upgrade policies you can by using the following. The example upgrades email@example.com to Teams Only mode.
Grant-CsTeamsUpgradePolicy -PolicyName UpgradeToTeams -Identity firstname.lastname@example.org
Once ran you can look at your user and you will notice the user has the policy now assigned. (This no longer is the case, it will just show blank now, until they update the GUI to support TeamsOnly again)
To set your entire tenant to Teams Only you can use the following command
Grant-CsTeamsUpgradePolicy -PolicyName UpgradeToTeams
How to access and use these commands
If you don't know how to access these Powershell commands you need to do the following.
1. First Install the Skype for Business Online powershell module from here
2. Says a reboot is required after installation but I don't think it is, try if it doesn't work, then reboot ;).
3. Now open powershell and run the following to connect:
$sfbSession = New-CsOnlineSession
After you enter your password to connect, you will now have access to all the commands to manage this and many other items with Skype/Teams.