Well, the first thing is that [URL="http://dirtsimple.org/2004/12/python-is-not-java.html"]"XML is not the answer"[/URL]. If you need to allow people to edit the file by hand; or if you are supporting an existing system, then maybe. Otherwise, I strongly suggest you use something else (pickle file? Python format config file?, simple ini type layout?).
Assuming you are stuck with this, I would either use an xml parser or a regular expression to find and replace the guid value. Be aware that since this is a text file, users [B]can[/B] hand edit it, so you cannot count on the exact format (there might be an added or missing line break, extra white space, comments ...)