Hi Nick,
That's right; you'll need to choose one of the mapping options for action to be taken on new and existing leads and contacts. We're looking into this, but as of this post, please select "overwrite all" or "only fill in blank". For what you want to do, I'd select "only fill in blank". It won't write over any existing data in the system.
Thanks for using the forum and please let us know if you have any questions!