How do you change what a function in a core module does?
This is actually pretty easy.

All you have to do is implement your own template_preprocess_node().  Then inside, it update the $variables that the RDF module made.  If you want to update the header additions that it did, then implement a hook_html_head_alter(), and update those elements.

If you do this from your theme, then these will run after the RDF module.  If you do it from a module, then you need to set the weight to be higher than the RDF module.