@Sidetrack - correct, this plugin has no ability to update metadata for an existing book.
That is the second time this week it has been mentioned though, so I'm sure it might be a feature one day. I just haven't had time to give it any thought as to how that might actually work from a UI perspective, as it is potentially complicated to manage. The next release I am working on currently allows a variable number of fields to be imported. Somehow the user would need the option somewhere to choose to overwrite data for an existing book...