As noted in the Message Center post (MC211416), there will be some admin options rolling out. These let an admin show the toggle for the new experience, and then decide if the user should be opted into the new experience by default. Additional docs will be available soon. There are no plans for PowerShell or Yammer API control over this toggle.
Please be patient as it takes some time for this to roll out to all networks in the early weeks of May. We are excited for you to launch this to your users!