I thought that Google deploys new versions gradually (first to 1% of users, and if that doesn't show errors, to more and more). Which implies that there are at

I thought that Google deploys new versions gradually (first to 1% of users, and if that doesn't show errors, to more and more). Which implies that there are at least two version of an application or service running. How does that work when you don't keep APIs stable, at least at the service boundaries?