locate *.m3u > myplaylists.txt
cat myplaylists.txt | while read line; do mv “${line}” “${line%.m3u}.pls”; done
Ouch thats complicated and requires findutils and wont work with busybox,
What about
rm `find /music -name “*.m3u”`
And if that gives a line too long error you can still do a man find and look for the -exec command of find. With the -exec you can also construct a mv then but as you said most werent needed anyway.