Changing date format in filenames in Linux

Evenin' all. (forum members under about sixty will definitely not get the reference).

I've got a lot of files in the format dd_mm_yyyy.mp3 which I'd like to amend to yyyy_mm_dd.mp3.

I'm sure I once found a way to do that using the command line, but I can't find it now.

Any help appreciated.

Mind how you go. (see above).