How to downgrade my mdf file from version 661 to 655 or earlier?
Simple answer: you cannot do that. 

There's no tool, no undocumented way, no hack, no workaround to achieve this. SQL Server database files just aren't "backwards" compatible - you can easily go from 2005 to 2008, or from 2008 to 2012 - but you cannot go backwards. 

You'll need to find another way to do this, e.g. by scripting out database structure and/or data into .sql files and running those on the old system.