After some light retouching I would like to save an image in photoshop over the original one (overwriting it). Is there a way to do that without modifying the original EXIF data?
You can export the data using exiftool and then write that into the modified picture - this is the easiest way.
I can refer you to the man page of exiftool, section "COPYING EXAMPLES". There are many ways to do this actually. One simple way is to create a backup of the original picture (orig.jpg) create a modified version (modif.jpg) and just use:
exiftool -TagsFromFile orig.jpg -all:all modif.jpg
When you are satisfied, just delete orig.jpg, and rename modif.jpg to orig.jpg.
As always, make backup of your files before messing with them with exiftool. (Verify that your metadata is properly updated.)