Sonar has a good track record of supporting moving forward with projects, so a project developed in 8.5.3 will open in either X1, X2, or X2a. It does not work well to then try to go backwards, but in any case:
1. If you have concerns about a particular project, simply copy the existing project folder and contents to a new project folder, and internally rename the project file to some variation on name from the old one.
2. Once that is done, simply open the new one in whatever newer version of Sonar you wish, and have at it. Your original project is safe and secure under its original name.
I have never heard of these 'monstrous stories' you referred to - perhaps you can elaborate. There certainly are things one must do if moving older 32-bit plugins to newer 64-bit ones, but that is still quite workable.
Bob Bone