Microsoft Teams is now a complete meeting and calling solution

I can provide more information on this now. The email we received was for the test environment and we can now see official message in the message center of that tenant. I believe small / low use tenants have this message now in their message center, so check your message center. I was confirmed from the Microsoft Support that "tenants scheduled for automatically update as all eligible customers will receive upgrade notification on Message Center and also in email." 

I have asked if we should expect to see this message soon in our production tenant and the answer from Microsoft Support was that they are "notifying customer and also sending message center and still if someone misses out and there tenant is upgraded, they can connect with support with business justification as why they don’t want to update and we can help them moving back to SFB and then they can plan their migration." 

So it seems that you can opt-out but you have to contact support. I have no idea what is schedule for production tenants and I my personal opinion is that after all they do not have the guts to do it automatically to production tenants... it would be unthinkable...