See:
http://www.archicadwiki.com/ArchiCAD%20versions
Near the bottom is a table of what versions each version of AC can read and write. As TMA says, 11 can save back to 10 (12 to 11, 13 to 12 etc.)
When back-saving, not only would you lose any info related to new features, if utilized in the project, but library parts cannot be saved backwards or opened in older versions.
Sorry,
Karl