there is an easier way but i wont recommend using that, because that changes the created/by and modified/by ... any wasy the method is
-) go to your document library
-) actions -> open with windows explorer
-) copy the forms to your local system
-) delete the forms form the original folder
-) paste the forms back and close the window
-) refresh your page and the new property fields will contain data
before going through these steps, do take the necessary backup ..