Changing the extension does _not_ switch the container format, although it might change the icon and it probably starts a different program when double-clicking, since on Windows files are associated to programs by their extension.
of course you are absolutely right. i should have mentioned, that in this special case (mjpeg codec with NO audio track), this quick and dirty method did the trick just fine.
a more proper way to achieve this would be a conversion with FFMPEG. i did that as well (for batch file converting) using the following command:
ffmpeg -i xxx.mov -c:v copy yyy.avi
which seemed to do just the same thing. any image-codecs like "PNG", "TIFF", "TARGA" etc. might work like that, since they are basically nothing more than a series of still images.
of course this method would NOT work if the original codec was "animation" or "mpeg2" etc...
then you would really need to re-encode the new .avi file. sorry for the confusion.
best
oliver