If you are want to keep the locations of the fields mapped but you want to swap out the PDF, there is an icon in the top left corner. Swapping PDFs is helpful when the version of a PDF is updated but the field locations have not changed. You can also swap out larger versions of a PDF to include addendums or additional location pages while keeping the initial section mapped.
Hit the Swap PDF Icon in the top left corner
Hit Continue
Review your swapped PDF and hit Swap