You have to 1) save the old searchable pdf to tiff images (as many as the pages), 2) ocr the tiff images to a searchable pf 3) combine the new multiple pdf to one pdf.
manage pdf online
Excellent websites are https://www.sejda.com/ (with the only limit of three works per hour, regardless to the pdf pages number) and https://tools.pdf24.org/ (with no limitations, but with less features than the first one).