I can see this thread is not moving anymore. But I have the same problem now miving a project from 8.6 to 8.0.I have the option to save with previous version (and it is documented) but It answers "Saving failed for an unknown reason" or whatever else. It is possible to save single files, bu that is all. Moreover, file is not simply saved, but it is saved in a separate directory, in such a way it is difficult to save many files in version 8.0 and then try again with the project.
Please post in a new thread as this one is archived. Can you post a screenshot of the error you are getting when you try to save in 8.0? In addition, when you try to saev in a previous version files are always saved in a spearate directory with the name containing the version you have saved in.