Here is a tutorial on how to save backwards in After Effects.
When you have your project open, navigate to the top left of the menu and click File / Save As / Save a Copy As CC (XX)
Once that is selected, it will open a save window that will let you choose where you'd like to save your files.
When you do this, AE only saves backwards one version of CCxx at a time. So if you are in CC14, it will save backwards to CC12, and then you will need to repeat the process in CC12 to get back to CS6.
To open up an AE file in an older version, simply right click the file, then select Open With / Adobe After Effects CC XX and choose the version number you'd like.
The image below only shows one option, but IF should have several versions installed. This specific machine I am on only has CC 2015.
Hope this helps! :)