1. PAF's "Global Search and Replace" function (but it does not seem to apply to sources)
2. GEDCOM tools that would allow you to do a global search and replace in a GEDCOM (but I have found nothing that will do it)
3. Opening a GEDCOM file in a word processor or text editor so you can use its built-in global search and replace function (but any source name that spans two lines of the GEDCOM could not be done that way).
Even if possibilities 2 and 3 successfully changed the name of the source, they might cause other problems. I don't know. And before experimenting with them you would need to create a good backup and keep it safe. Any newly-created PAF file should have a different name than any of your current PAF files so that it doesn't replace an existing file.
So I really do not know of a solution, and you do not want to do anything that would jeopardize your research.