[root@localhost pluginconf.d]# yum --allow-downgrade localupdate /var/local/crds_repo/ruby-1.8.5-3.fc6.i386.rpm
Loading "allowdowngrade" plugin
Loading "installonlyn" plugin
Setting up Local Package Process
Examining /var/local/crds_repo/ruby-1.8.5-3.fc6.i386.rpm: ruby - 1.8.5-3.fc6.i386
/var/local/crds_repo/ruby-1.8.5-3.fc6.i386.rpm: does not update installed package.
Nothing to do

[root@localhost pluginconf.d]# yum --allow-downgrade localinstall /var/local/crds_repo/ruby-1.8.5-3.fc6.i386.rpm
Loading "allowdowngrade" plugin
Loading "installonlyn" plugin
Setting up Local Package Process
Examining /var/local/crds_repo/ruby-1.8.5-3.fc6.i386.rpm: ruby - 1.8.5-3.fc6.i386
/var/local/crds_repo/ruby-1.8.5-3.fc6.i386.rpm: does not update installed package.
Nothing to do