Amend a book in PDF - typesetting.

This is a very specific typesetting/technical task.

We are an academic publisher, working with an (older) author who has a book in PDF which was previously self-published. The book itself is long (600+ pages) and the existing format is ok, but not perfect. At the very least, the existing front matter needs to be replaced with new front matter (which we have). The author does not have any reliable native files (Word, InDesign etc) that the PDF was created with. It's not an old PDF (maybe 2 years).

We need a solution, which needs to be either:

1. Just replace the front matter
2. As 1, but sort out a few glitches in the body text
3. Back-convert and re-flow (I don't even know if this is possible, and may be way too much trouble, as it has extensive footnotes).

We have all the necessary files. No great rush, but we would like it turned round within 2 to 3 weeks if possible. More details on request.