A way to batch copy contents of one field to another field?

I have a lot of classical mp3's with info in the composer field that I want to mass transfer to the genre field (my mp3 player can read genre, but not composer). I was trying to make an action using

Replace "GENRE": "%genre%" -> "%composer%"

but I don't think this is how the syntax or the intent of it is supposed to work (it didn't work).

I was also thinking of just sorting by composer and copying and pasting the info over by hand, or using some combination of that and tag->filename and filename->tag, but was wondering if there were a tidy way to write an action for it instead (the other methods could introduce mistakes, or with using the filename as a temp holder for the tag info, could run into problems with filename length, illegal characters, etc)..

Thanks!

https://docs.mp3tag.de/actions/format

DD.20090816.1045.CEST

Thanks! I'm sorry I missed that -- I guess I didn't look hard enough, so, I definitely appreciate it..