This certainly isn't my wheelhouse, but you might want to elaborate a bit on situation. Is this a single server environment? Which conf file was overwritten? How do you plan to get it back by downgrading? What is the actual state of your environment at the moment?
I mean, sure, you can uninstall 6.4 and install 5.0. But without knowing what problems you're actually trying to address from the upgrade, I imagine it will be hard for others to advise you on whether those steps make sense.