for file in /dir/*.123 ; do
ln -s $file `echo $file | sed 's/123$/321/'`
I haven't tested it but it should work no probs! Just make sure you change the right things in it
(if you wanted to change mp3 to ogg you'd change s/123$/321/ to s/mp3$/ogg/ and /dir/*.123 to /dir/*.mp3)
edit: added code tags, whoops!