Taking inspiration from the Linux
ipsr and the MPEG4 Mod programs, a friend has written the attached KiSSconv utility.
The utility will check if the file needs the change in the User Data from DivX503b1393p to DivX999b000p and make the changes on the fly, directly in the avi file.
Run it either from the command line, with the filename (full filename, for example Blahblah.avi) as the parameter (if you want to see how many times it changed the string in the file, which is the output of the utility) or just drop the avi file onto the executable (which will open a cmd window but that will close once it has finished processing the file).
Attached zip file has both source and exe.
WARNING
Use with care as the utility modifies the file in-place, thus as your original file will be directly modified there will be NO backup copy of it. Therefore please make a backup of the avi file before running the utility if needed/unsure/etc. I have been using the utility and it has never corrupted a file, but just in case...