If they are DRM-free, then mobi2oeb (from
Calibre) can explode the MOBI to HTML and images and a .opf file. Or you can use mobi2html (from
MobiPerl), although it does not work on MOBIs with high compression. You can then edit the HTML and recreate a MOBI with Windows MobiPocket Creator (or Reader) or MobiPerl's html2mobi.
Some of the existing metadata may be lost in this process, although the .opf file from mobi2oeb contains some of it and if the HTML has not changed much you can import the .opf file "as is" into Creator or Reader (providing the new HTML has the same filename as before).