In the last times it happens more and more that I need to exchange file with users that are still with older versions of Archicad. And every archicad upgrade there will be a step "open and save to previous version" more. And the need to hold more versions of Archicad on the disk... and the problem will affect loyal users above all.
I think that Graphisoft should review the politic of "single version back save" or create a conversion tool.