Hi @Jorgenson, Brian E ,
It will affect Skype for Business on prem users communicate with Teams users when you set Teams organizational default config from Islands to Teams only mode. The following table show that you can only use Microsoft Teams to chat each other.
You can change the upgrade options for all user at any time in Teams admin center. For more reference:
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Hi @Jorgenson, Brian E
Do you have any further issue on this topic?