The functionality to make it possible to change the name of an O365 tenant, especially with Sharepoint Online in mind, is being planned. There is no ETA at this time though.
Thanks to everyone for your feedback and votes on this item. We’re working through a couple of options and hope to have more information shortly. It’s a very difficult problem to solve since group membership is consumed across many different aspects of Microsoft 365. Thanks for your patience and keep the feedback coming!
“Secret groups” is still on the Microsoft 365 Groups backlog, but there is no delivery date available at this time.