I dont recall ever being able to save a Premiere Project back into a previous (legacy) one.
My current and previous computers (and back ups) are testimony to this.
For every version of Premiere back to CS2 I have a 'Version_projectname ' Folder where the Projects are saved.
Projects for all applications have a dedicated drive for this purpose. (D/:PROJECTS/ APPLICATION Version NAME / "project name.
Currently because CC2014.1 ( a dot version !) is not backward compatible... looks like D/:Projects/ Premiere CC8.1/ "project name".