There exists many different codecs that basically implement the same compression algorithm. One example of this is the mpeg4 algorithm which is implemented, with different levels of enhancements, by the divx, xvid, angelpotion and h264 codecs (and many more).
To some degree it is even possible to decode a video stream encoded using one of these mpeg4 codecs with another mpeg4 codec. This can be usefull when the video file was encoded with a video codec that is hard to find or with one that is highly unstable.
The AVI FourCC Code Changer allows you to change the FOURCC in an AVI file. It does this by modifying the file so you should be very carefull when using this piece of software. Backup the file first!