For existing forms, you'd need to set the content type as the default for the library, then navigate to the Relink view in the library, and relink your existing XML. Relinking updates the href in the InfoPath processing instructions in the XML (download an XML file, take a look in the text editor of your choice and you'll see the href). Warning - relinking ALSO changes the Modified date & Modified By - not a deal breaker for most people, but I have run into some who care. So - relink a single form, and verify things work as anticipated. If you set your content type to the default, when you click the New Document button, that will be used automatically - I usually make it my only content type. Another benefit of this is if you make major revisions, you can publish as a new content type, and all the existing forms will continue to use the old form (this can sometimes be a real PITA in InfoPath, when you want to make changes but don't want your existing XML to reflect those). I encourage you to set up a quick test form / libraries, and try some things outside of your production environment (I'm guessing all this is live, based on you having existing XML?) so that you have a little freedom to try some things without worrying about breaking production. :)
Hilary Stoupa
