There's a bigger problem though: in practice there's almost always a client that you don't control, and can't switch along with your services, e.g. an old frontend loaded by a user's browser.

But (in my mind) even a front end is going to get told it is out of date/unusable and needs to be upgraded when it next attempts to interact with the service, and, in my mind atleast, that means that it will have to upgrade, which isn't "atomic" in the strictest sense of the word, but it's as close as you're going to get.