This is not how it works. The next release name is announced approximately when the freeze starts. When a release happens, oldstable is archived, stable become the new oldstable, and the current state of testing becomes the new stable.

For a moment of time (we're talking hours), testing and stable have the same set of packages. Britney, the software which migrates packages from unstable to testing, runs four times a day. During a freeze, it is configured to need "unblock exceptions" to migrate packages. When this is option is disabled, the following britney run will make the first difference between testing and stable.