Microsoft has begun alerting some customers about proactively migrating them from Skype for Business online to Teams. On the preset target migration date, Microsoft executes the migration automatically.
Customers wishing to delay the move must respond to the alerts. The alerts come in the Weekly Digest email that is sent to the delegated administrator(s). The alerts also appear in the Office 365 administrative Message Center under "Health".
It's very important to pay attention to these alerts, because after these notifications, Microsoft will automatically migrate customers, likely disrupting their user experience. This migration can be delayed by communicating back with Microsoft. Microsoft is currently focusing on customers with fewer than 500 Skype for Business online subscribers.
Bottom line, please pay attention to your Weekly Digest and administrator alert pages for this and other important information about your tenant.
Microsoft’s official notice is posted at https://docs.microsoft.com/en-us/MicrosoftTeams/upgrade-automated