Due to the way the archive works, that doesn't really happen.
First of all, you should know that release n+1 is seeded from release n when it is opened for development. Roughly, this means that release n is copied and renamed to n+1 (there is some tinkering that happens before it is opened for development, but that's not important here). Thus it begins with the same package versions as release n had.
Now, in order to upload you must use a higher version than was uploaded previously. This means that you cannot 'go backwards'. Even if this were allowed by the archive, the package managers on your system wouldn't consider it to be an upgrade and so you'd keep the 'newer' version.
If a package gets taken from unstable in release n and somehow this version never makes it to testing then we will usually stay with the version in unstable for this reason.
Having said that, it is possible to hack the version numbers a bit to allow reversions — you may have seen 'foo-1.0+really0.5' versions before — this is a rare way to pretend to the archive and to your package manager that the versions have increased when in fact the upload contains an older version. This is only used as a last resort when a particular version of some software has problems that are so serious that a rollback is the only option. It's far more preferable to just fix the problem directly. Also, this is orthoganal to the release/LTS system — it can happen at any time.
So in summary, it's possible to revert packages to a previous version by using package manager hacks, but this isn't generally done.