We received notification of the new navigation via email on Mar 15.
On Mar 20th we got a banner indicating it was available for use, so I turned it on as an admin, for myself to take a look at the changes.
At 4pm on Mar 20th, the new navigation was pushed out to all users sitewide (Jira Service Management Cloud, Standard). I've checked with the others who hold admin rights and confirmed that they did not enable this.
Not overly happy with the speed of the rollout, I was of the understanding that we would have time to evaluate and send comms to our users to let them know about the upcoming changes.