What is the result of turning off Skype for Business on-prem without migrating all users to Teams considering that these users use Teams in Island Mode? I know that this is not the supported way of moving from S4B to Teams, but I want to understand the impact of a “migration” done this way. I looked in the documentation about the upgrade but I only found mention of not having “all the features” of Teams. In this hypothetical scenario, S4B would be turned off and the DNS changed to Teams. What happens in that case?
If you have enabled SFB Hybrid with shared sip address space to SFB Online, then a proper migration of moving the users from on prem sfb to TeamsOnly is required. If you have users hosted in Skype for Business with full telephony features, auto attendants and call queues that will be a major impact. And users will lose their meetings hosted in Skype , their Skype Contacts and favourites as well.