[KevinH]

Under epub3 the modification date and time is considered part of the book identifier so that duplicate uuids need not be a worry allowing multiple revisions of the book to keep the same base uuid.

Also, be careful to turn off any and all font obfuscation before making any uuid change and sync it with the ncx metadata, and make sure not to mess with any id attributes on the dc:identifier in case it is referenced in the package bookid attribute or in other metadata (under epub3) and to set the scheme properly for epub2 vs epub3.