Option to delete/reset values in hidden fields on condition change?

I have a controlling field that permits a user to either submit a link or upload a file. This works fine, except that if the user first enters a URL, then changes her mind and uploads a file instead, the originally entered URL remains in the node. To avoid this, the user has to blank out the URL field first, before switching the controlling field to upload a file. This is counter-intuitive--if users no longer see the field, they aren't going to expect to be submitting data in it. Is it possible to reset a field to its default value, or remove its value, when the controlling field is changed?