I have received feedback from serveral readers of PDF-AKTUELL that the installer of Acrobat DC de-installs older Acrobat versions. This especially true for subscribers of the Creative Cloud since Acrobat DC is installed as update of Acrobat XI. (For other CC applications (e.g. InDesign) new versions are installed along the older versions. But not for Acrobat!)
At the beginning Adobe did delete the old version without any warning. After a lot of protest in the Internet the installer now displays a warning before deleting. But it’s not possible to skip the deletion.
Since on the Mac the plug-ins are stored in the package of the Acrobat application (show package content with a right mouse click) all third-party plug-ins get also deleted together with the old Acrobat software. That’s why you should make a backup of your plug-ins folder before you install Acrobat DC.
Be aware that you cannot use old third-party plugins on the Mac. But it’s good to save a copy in case you want to revert to the old version of Acrobat.
Before the installation on my Mac I have renamed the Acrobat XI folder (and to be sure also made a ZIP archive of this folder). With this I could fool the Acrobat DC installer.
In case you did loose the old Acrobat version you can try to re-install it. Here you can find the installers for Acrobat X or XI and Acrobat 9. Adobe does not recommend to install more than one Acrobat version on Windows. (Acrobat and Reader on the same computer are OK.)
In case the old Acrobat version should be removed by Acrobat DC one can try to re-install the old version and the third-party plug-ins. Here you can find the installers for Acrobat X or XI and Acrobat 9. On Windows Adobe does not recommend to install multiple versions of Acrobat on the same system. However Acrobat and Reader are OK.
Adobe has published a help page for regular users and one for Creative Cloud users.