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

Or to write them to another directory, keeping the original structure intact:

cd /dir/to/convert && fd -t f -e flac -x bash -c 'D=$(dirname "{}"); B=$(basename "{}"); mkdir -p "/dir/to/write/to/$D"; ffmpeg -i "{}" "/dir/to/write/to/$D/${B%.*}.opus"' bash {}

via https://quantixed.org/2021/11/20/convertible-using-ffmpeg-to-convert-audio-files/

^ Nope, that seems to miss files…

You should probably use beets or something…

soundKonverter is a nice tool too.