For some time now Microsoft has made no bones about the fact that Microsoft Teams is the future replacement for Skype for Business (S4B). In fact there isn’t much you can do in S4B that isn’t already in Teams today.
By default, you have S4B and Teams side by side and most organizations don’t change this at the onset. Fortunately Microsoft has now published an article to Understand Microsoft Teams and Skype for Business coexistence and interoperability that can explain all the options on how this can work. Note that best practices recommend a controlled rollout of Microsoft Teams, once a usage guidelines and governance strategy has been defined for the organization.
The issue that most organizations have is how to migrate their users over to Teams. The default situation means that most users will continue to use S4B until they find the people they want to communicate with are already communicating on Teams. At that point they will use both until all the people they communicate with are migrated.
This is generally okay as most organizations want some pilot or leading edge users to try it out before migrating everyone. The trick here is that you may want to control who has access to Teams until you are confident or comfortable enough to migrate everyone. This means a controlled rollout of Microsoft Teams, a topic intimately tied to how you manage Office 365 Groups and has been discussed and documented well in the last year. Once the pilot is complete, then you can formalize a migration plan consistent with the usage guidelines and governance strategy.