Extensions

I have been advised to change MP4 extension to M4A in my collection...(something to do with compatibility in iTunes, iPod).
Trouble is I have about 4000 MP4 files.
Can any of you guys tell me how to effect batch changes of this type?

Cheers

Action: Replace with Regular Expression
Field: _FILENAME
Regular Expression: .mp4$
Replace Matches with: .m4a

Pone, you are a star!!

Thanks