Great news, but I think the ability to only Groupify an entire site collection and not individual team sites within a site collection will be very limiting to us.
I'd consider a data migration to new Office 365 Group sites, but we have issues with lists which are used across different team sites for lookup columns. BCS isn't an option given we would want to move/copy documents between groups (can you even do that?) with metadata intact and they are multi select lookup columns. FYI These lookup columns are populated from an on-premises SQL Server using the SharePoint API. In hindsight we should have used the global term store and found a way to populate and update it with the API (if even possible?) and setup the content type in the content type hub, but obviously we had no idea Office 365 Groups would be coming down the line.
I suspect what we will do is create Microsoft Teams enabled Office 365 Groups, but keep using the existing team sites in addition and just provide navigation links between the two.
I would be interested to hear from anyone else tackling similar issues.
Lawrence