The easiest way to transfer documents in Paperless is to simply select the items to transfer then drag and drop them into the new library.

If you have custom fileds, document types or such, you may consider creating a "clone" of you existing file. When you make a new Libbrary file (File>New Library...) you'll see the option to base the new file on an existing file. Select the exisitng file and the new file created will have all of the "structure" of your old file - just none of the content.