Hello thanks for the quick reply.

I tested it out on a 10 page document that just has 10 images linked in the header. No other content in the document. All the files are called preview***.png X being numbers. I moved all the jpgs to the same directory as the png files. All the files are exactly the same, they are just JPG files instead of the old PNG files (the jpg files are smaller in filesize).

Saved the document as a doc file, checked out the links before (all png) ran the script then looked at the links after, still png files and not the jpg files.