I agree that there can be problems by saving a report to a newer version, in my case I sometimes need to open and work on a report in specific version, and save it to that same version.
I have v7,8,8.5,9,10,11,11.5 currently and am trying to figure out which ones can co-exist. I thought that some of the earlier versions used the same DLL names, hence if I install v8 that I would have problems trying to use v7.