In my books I recommend that you NEVER open a previous version's project with a newer version of the program. It almost always leads to file corruption.
I recommend you finish your version 8 projects in version 8 and then start fresh in version 11.
Welcome to the forum.
Unfortunately, the PREL (Project) files are not X-platform between the PC and the Mac.
This is just speculation on my part, but I believe that those differences are between the included Sonic AuthorCore modules - the sections that allow one to author DVD's and BD's. PrE contains the capability to do authoring, and the necessary info is included in the PREL. With Premiere Pro (PrE's big-brother), the Project files, PRPROJ files, ARE X-platform, so long as they are from a version of PrPro, that is X-platform - every version from CS 3 to CS 6 - PrPro 1 & 2 were not X-platform. Now, as of PrPro CS 3, a separate authoring program, Adobe Encore, was included. The Encore Project files, NCOR files, are NOT X-platform. With PrE, the authoring capabilities are built in, and I think that is where the issue stems, but as I am not an Adobe engineer, I can only speculate, based on PrPro and Encore.
Even if the PREL X-platform incompatibility did not exist, and as Steve mentions, finishing a Project in the exact version, where it was started, is always the best workflow. Even if one is not also changing the platform, there can be problems, as features are added, and removed, from version to version. Some Projects WILL translate to a later version, but the more complex the Project, the less likely that everything will come over properly. Often, there are little glitches, though some can be overcome - some just bring the Project to its knees.
Sorry for the bad news. Is it possible to finish your earlier Project(s) on a PC? If so, that would be my recommendation.