The "downgrade" is "downgrade rights. When a new version is released, such as SQL Server 2014, that is all that's being sold. When you buy the license, however, you can install any previous version and use that 2014 (very expensive) license against a 2012, 2008, etc installation. This is administrative only.

If you install 2014, then you are stuck with 2014. You can set compatibility to 2012, but that just affects how the engine interprets the code and manages stats. The structure of the files is 2014, the backups use any changes 2014 has, etc.

If you want to have 2012 or 2008 instead, you have to do as Grant noted. Uninstall 2014, reinstall the version you want.