How can I convert my music?

I have a fairly large music collection, which is 9.9 GB in size. It's mainly made up of MP3 files, with some OGG Vorbis files and a handful of WAV and WMA files. I would like to convert the entire library to AAC (or a better format, if there is one) in order to reduce the size of my collection by a considerable amount.

My library is organised using this folder structure:

~/Music/{Artist}/{Album}/{Track}

Can anyone recommend a GUI tool or shellscript which would recursively convert the files, map across the metadata, and dump the files into a different folder with the same directory structure?

EDIT: I have used a script to convert everything to Opus. Problem solved, just working out the kinks now.