Adobe does not always include a "save as" that goes more than one version back... especially not back to CS6 which is going on 4 years old and is no longer supported
AE has never been backwards compatible, so be thankful for what you have. Even that took a lot of effort.
What they use to do and what the should do are 2 different conversation. They are pushing to be the industry standard in all things digital, so giving the option to back save should not be that hard to accomplish. The files could possibly be exported to Media Encoder and back saved, or an addon/plugin that allows for this to happen. If I can accomplish this task by installing CC2014 and CC2012, there is no reason for there not be an option for this. Not everyone can afford a monthly fee especially when they paid the large upfront cost of CS6.