I would like to add something about working with projects.
I have always had the philosophy that I would finish projects in the same Sonar release I created them in, and only create new projects in the new Sonar release. Some folks share this approach, and some folks open projects created in prior Sonar releases and don't worry about it.
In any case, if you keep X1, and then install X3, you have the choice to either continue to work with the X1 projects in X1, or you can open them in X3. Newer projects could be created in either, but I would think the obvious choice would be to create the new ones in X3.
As was pointed out, keep the plugins that were dropped, like V-Vocal. Beatscape was dropped, but I think its sounds were rolled up into something else (cannot recall what, because I didn't use it).
At one point, I had Sonar 8.5.3, X1, and X2 all living happily alongside each other. I now have X1 and X3 and that's it.
Hope that helps - you will LOVE X3, by the way. It has great new features, and is very stable, and FUN.
Bob Bone