Well, subject says what my question is.
Run this command to figure out the names of the tags you want to copy to and from
exiftool -a -s -G -RequestAll 2 /path/to/files/
Then you would copy the source tags to the target tags
exiftool '-TARGETTAG<SOURCETAG' /path/to/files/
The tags you want to change may not be writable by exiftool, though.