Hi Folks,
   I tried to solve this a few days ago under "Opens in X2, not in 8.5",---but I have a little more information now. I have both X2 Producer and 8.5 Producer installed in my machine running an AMD quad-core with 4GB of Ram and the 64-bit version of Windows. I have worked on a project in X2, and saved it as a .cwb file and as a "normal" file (.cwp, I assume). When I go into 8.5 and attempt to open the .cwb file, I get the following message:
"File Info: Project file version 10.0 authored using Sonar version 19.0.0.306. This file was saved from a more recent software version. It may sound different from the original if new features were used while authoring it. Overwriting this file with the same name will cause the loss of these features."
   If I try saving the file as "normal", I get an error that there are missing audio files. Please bear with my ignorance,---but I have no idea how to open this tune in 8.5. Any help will be greatly appreciated.
 
Chuck

Hold down CTRL and then click on the CWP icon or from the file menu. 
Holding CTRL opens in safe mode. 
I just did this. As I too have both X3 and 8.5 and get the same message when going backwards,, and sometimes forward. My version actually told me to try this when the file wouldn't open because I believe I still have screen tips checked. 

The SONAR version is saved along with the project, so if it's subsequently opened with an older version you'll get this warning. It doesn't necessarily mean you can't open the project. In fact, you usually can. If you didn't  use any X2-specific features such as ProChannel, it may open without any problems at all.
 
What's most likely happened is that Windows still associates the .cwp file extension with SONAR 8.5, so double-clicking on a project file opens 8.5.
 
If you right-click on a cwp file and select "Open With"->Choose Program, you can choose X2 and check the box that says "always use the selected program to open this kind of file". From then on, X2 will open when you double-click a project file. If you do want to open an older project in 8.5, start SONAR first and then open the project.