Hey @Rather,
The exiftool Unix Command-line tool makes this relatively easy to do from the Terminal.
cd "~/test_directory/exif_test/";
exiftool "-filemodifydate<datetimeoriginal" "-modifydate<datetimeoriginal" "-metadatadate<datetimeoriginal" "-historywhen<datetimeoriginal" -overwrite_original *.jpg;
Here's a relevant thread:
What kind of files are you managing? I've limited the shell script to jpegs, but that is easily altered.
Make sure you test on COPIES before working with the originals.
-Chris