How do you know when your tenant has fully migrated to new Stream (on SharePoint)?

akaTiggers - We are still developing the migration tool. Currently it's in private preview and only a small set of customers have it enabled for testing. Later this year when it's open for all customers to use there won't be any "automatic migration". Each customer will have to use the migration tool to move their content out of Stream (classic) and into SharePoint / OneDrive. 

Right now a customer needs to determine if they want to keep uploading videos to Stream (classic) or start putting new videos into SharePoint, Teams, Yammer, OneDrive and dual use both solutions in the interim. 

So if you want to use the new path Stream (on SharePoint) you can do it now by just putting your videos into SharePoint, OneDrive, Teams, Yammer instead of Stream (classic). Lots of great features are on the verge of being released in the next few months (transcripts for videos in english, chapters, and comments). And several more features we'll be adding to the public roadmap in the coming months like custom thumbnail, a new player page that looks more like Stream (classic), embed codes, more transcription languages, etc.