1. Create a new storyboard
  2. Cut the View Controller out of the old storyboard and paste it in the new one.
  3. In the new Storyboard, in the Attributes inspector, set “Is Initial View Controller” and change the Bottom Bar to Opaque Tab Bar
  4. And in the Identify inspector, set Storyboard ID to NewStoryboard.
  5. In the old storyboard, drag out a Storyboard Reference from the Library.
  6. Control drag from the source View Controller to the Storyboard Reference and select “Relationship Segue > view controllers” from the popup.
  7. In the Storyboard Reference’s Attribute inspector, change Storyboard to an item from the dropdown (used NewStoryboard in this example).
  8. Unfortunately the Nav Bar Item icon on the old storyboard appears as a square with the text Item. Haven’t solved that yet.

I thought I missed “Editor menu > Refactor to Storyboard…” but it will only take 1 view controller with it.

As of Xcode Version 11.6