By default, when you install a CC 2015 application using the Creative Cloud desktop application, previous versions of that application are uninstalled. The default installation leaves only the most recent version. For example, installing After Effects CC 2015 (13.5) using the Creative Cloud desktop application uninstalls After Effects CC 2014 (13.2) and After Effects CC (12.2).
To retain these previous versions during the installation of CC 2015, deselect the Remove Old Versions option in the Advanced Options section of the updater interface. See this page for additional details on installations.
For more information on reinstalling previous versions, see Download your creative cloud apps.
Because After Effects CC 2015 (13.5) is a complete rearchitecture of the application, most plug-ins need updating to work with this new version. If you require a plug-in that has not yet been updated, use the previous version of After Effects. Use this version until the updated plug-in version is available. For more details about plug-in compatibility, see this page.
Collaborators or clients who have not yet upgraded:
Since you can save projects back one major version number from After Effects, you can still work with the most recent version for your creative work. Then, save backward when needed. You don't need to save backward to go between After Effects CC 2015 (13.5) and After Effects CC 2014 (13.2)—the project file format is unchanged, and the major version number is the same. For details about saving backward in After Effects, see this page.
Require features not available in the version:
With each new version, Adobe takes the opportunity to prune features that haven’t gotten much use. And, as is the case with After Effects CC 2015 (13.5), Adobe removes features because of a major architectural change. Even if you require a feature that is not available in After Effects CC 2015 (13.5), perform the bulk of your creative work in After Effects CC 2015 (13.5). With that work complete, open the same project in After Effects CC 2014 (13.2) for the part of your work that requires the removed feature. For details of features not available in After Effects CC 2015 (13.5), see this page.