Auto-Tune Pro (version 9.x.x) is compatible with all versions of Auto-Tune 8.1, but Auto-Tune 8.1 (and all previous versions) are not compatible with Auto-Tune Pro.
In other words, if you have existing projects saved with Auto-Tune 8.1, after installing Auto-Tune Pro, Auto-Tune Pro will replace previously saved instances of Auto-Tune 8.1 while preserving all of their respective settings.
However, if you save a project with Auto-Tune Pro, you will not be able to load Auto-Tune Pro settings with Auto-Tune 8.1 or earlier versions.
This is why we highly recommend that you work on a copy of your project files after installing Auto-Tune Pro. For example, if you have Auto-Tune 8.1 installed, be sure to "Save As..." any projects you wish to demo Auto-Tune Pro with before installing Auto-Tune Pro. Doing this will allow you to keep an original copy of your work with Auto-Tune 8.1 in case you need to access it later with Auto-Tune 8.1 instead of Auto-Tune Pro, or if you decide to not upgrade from Auto-Tune 8.1 to Auto-Tune Pro.
If you have Auto-Tune 8.1 and installed a demo of Auto-Tune Pro, but do not wish to upgrade to Auto-Tune Pro, please see these instructions to reinstall Auto-Tune 8.1.