Thank you for your interest. Infranview from tucows doesn't even want to begin to load. I have software for a Kodak camera and a Pentax camera, Adobe Photo Delux and Thumbs Plus on my PC. All pefectly adequate in their way but not entirely compatible in that pictures generated by one are not necessarily accepted by another. Whether changing the extension does anything to the internals of the file or not doesn't seem relevant as doing just that can make one program accept a file it has rejected before. Some for example will accept tif but not tiff, jpg not jpeg, jpg not tif and so on and so on. jpg seems the best compromise. I thought a program to batch change extensions was a feature of some later operating sytems, could be wrong. I am sure I saw such a thing on a recent magazine disc but can't lay my hands on it. To save any more time and space here I'll change each back individually.
As mentioned above, remaning does not change the content of the file but if you are trying to undo your 400 changes you can remane in DOS.
Put copies of all the files to be changed into a separate directory (Don't work on the originals). In a DOS window navigate to the relevent directory using the CD command. Rename using the command REN *.tif *.jpg
This command will rename all files with extension tif to the same name with extension jpg.