Very good question.
Let me explain by example. Let's say there is site A with 80 pages and you created a new version, site B and it has got only 40 pages. And you need to make new version B live.
See if you can only launch partial site, meaning load new pages and leave old pages which you didn't create. This can be a tough one if you haven't planned well.
Load new pages and for those pages which you haven't created, just create a basic content page with minimum info. It will ensure it doesn't go to a 404 in Google's eye.
Identify pages with higher authority/ranking which you didn't create and see if you can create important ones with basic info and then load and remaining redirect to home page temporarily and do remaining sooner.
Redirect remaining to the home page and create sooner so that Google will understand it was only a temporary redirect.
Note: If you redirect to home page make sure it's not permanent 301 redirects.
Hope this helps.