KBExit it will take time for everyone to upgrade. Many communities have modifications that probably will have to be made compatible, even if it's extensions that are still pending an update.
n-peugnet no there is no way to keep both versions in one branch. The code is too different for that. Even if something like this could be possible, the code would become such a mess that we won't offer a version checking extender from core.
You might be able to build one yourself though, but the outcome will be to have different files for each version most likely.
