FWIW you can solve this in a backward compat way by doing something like..
2013a -> 0.2013.01
2013b -> 0.2013.02
Or
2013a -> 13.1
2013b -> 13.2
While simultaneously releasing the same thing under the original versioning scheme. The one place this would fall down is if someone installs a "pre-release" of 2013a, but then attempts to only install "stable" after that. The "stable" versions will always appear to be less than the "pre-release" versions so people would need to be consistent in how they tried to install it.
FWIW you can solve this in a backward compat way by doing something like..
2013a -> 0.2013.01
2013b -> 0.2013.02
Or
2013a -> 13.1
2013b -> 13.2
While simultaneously releasing the same thing under the original versioning scheme. The one place this would fall down is if someone installs a "pre-release" of 2013a, but then attempts to only install "stable" after that. The "stable" versions will always appear to be less than the "pre-release" versions so people would need to be consistent in how they tried to install it.