To move or copy data between fields, go to Library Tools > Move/Copy Fields... (You can get to Library Tools several ways, including right-click when on one or more music files.)
Pick Source field, Destination field, and desired action: Move, Copy or (my fav) Flip-flop.
I recently used this to flip-flop Artist and Album Artist values for 70 thousand music files, with just one click (and plenty of time to make a sandwich while MC munched on the files).
If you're curious, I did the latest flip-flip in a never-ending search for the "perfect" way to organize artists by lastname, firstname but also preserve and display the recording's original "by" name, whatever it might be. My latest system is to put lastname, firstname in Album Artist, and the true exact "by" name in Artist. I used to do it opposite, hence the flip-flop, but this prevented the true original artist name from appearing in display mode during playback, on my iPod, etc. For instance, jazz musician Benny Goodman used a bunch of different group names -- Benny Goodman Quartet, Benny Goodman Sextet, Benny Goodman & his Orchestra, etc. Whatever it is goes in the Artist field, while Album Artist is always "Goodman, Benny".