CCB is still a work in progress, some things are about to change for sure in the future (that's why it's labeled alpha), depending on user needs, feedbacks, etc...
Big features like App Thining may not be integrated soon, because it would mean a lot of changes in CCB, like exporting all assets in image catalog, refactoring the way CCB handle assets, etc...
Even if it is, if you have a long developed game in your hands, there will not be enough killer feature in CCB that could make you want to upgrade your project.
Moreover, as I said, I think there is not be enough SB users now who may want to migrate to CCB in the near future. That allows me to clean everything and to make some necessary changes in some structures/files/method. Otherwise, thinking about backward compatibility is a big headache, and adds a lot of work. My time is unfortunately not infinite. I prefer working on future features than supporting past abandoned application (SB has not been upgraded from 1 year now, meaning I don't think a lot of developers started building their new games with SB recently).
If a lot of developers really need to migrate their project, I may take some time to do this, but it may ends up with a simple tutorial instead (using a migration tool is always dangerous, there is a lot of things that could turn bad if users changed some things from default project...)
For me CCB is not SB, that's why I wanted a new name. SB tutorials are mostly outdated out there, there are still a lot of confusion with different version of cocos2d/SB. I will make new tutorials up to date with current CCB development version.
If anybody wants to help for a migration tool, help is always welcome