Is there any way to alter the url of a FileField in Django?

[McKinley]

FileField objects have a url attribute. How do I alter it? The docs say it is read-only. Is there even a way to alter the underlying storage class? I am working with a migrated database and can't get the urls of the old directory structure to apply the new deployment. The MEDIA_ROOT is set fine, upload_to is being used in models, so new objects get saved correctly, but old ones are expected to be in the MEDIA_ROOT by the application, which is not the case.