No, there isn't a retro-converter.
I am afraid the winter will indeed be long .
There is no function or utility to convert a FileMaker file to a previous version.
You may try to copy tables, forms, etc. from FMP12 to an empty FMP11 file, but I am afraid that will fail somewhere. FMP12 introduced themes and just that will more than certainly fail.
You can move most many of a version 12 solution into a version 11 file as long as they don't use features not present in that older version, but you'd have to do it piecemeal and it would take quite a bit of work.
You can import or copy/paste table definitions. You can copy and paste layout objects. You can import or copy/paste script steps. Some elements have to be recreated "one piece at a time". And all of that limited only to features that work in FileMaker 11.