On an Office 365 tenant running Skype for Business, you may be “randomly” selected to upgrade from Skype for Business to Microsoft Teams. You will not miss this strategically placed violet coloured notification when you start the Skype for Business (desktop) application.
It is possible to ignore this upgrade prompt for an unknown duration until one fine day where you are “forced” migrated automatically in the cloud. This is evident when the all-too-familiar Skype for Business interface is replaced with “Your organization is now using Microsoft Teams” banner when you successfully sign-in as illustrated.
From this moment on, you are blocked from accessing all your PIM information from contacts to history list and key functionality such as the meeting calendar and voice phone dialer within the Skype for Business app. Administration of existing Skype for Business meetings prior to the upgrade to Teams can continue to be done using Outlook though.