I suspect that they uninstalled the old version prior to installing the new one. If they had just updated, both versions would have still been available.
I haven't tried this, but one possible option would be to include the versions of the Crystal files you app uses in the install for it so that they install in the same folder as your app. One of the beauties of the .NET framework is that you can have multiple versions of the same .dll file loaded at the same time. Applications look for the assemblies first in the application folder and then elsewhere on the computer based on information in the Global Assembly Cache (GAC). You can determine what's in the GAC, including the version number, by looking at the C:\Windows\Assembly folder.
-Dell