Because I just did it and might need to do it again, e.g.:
fd -t f -e flac -x bash -c 'ffmpeg -i "$1" -b:a 64k "$1".ogg && echo rm "$1"' bash {}

Remove the echo to make it actually delete the originals.

via https://wiki.archlinux.org/title/Convert_FLAC_to_MP3#Parallel_with_recursion