Changing File extensions

In order for the IPOD to recognise Audio Books properly i.e. as a book and not as music. It first has to be converted to MP4 format, using Itunes, giving the file an m4a extension, this is then converted by hand to m4b. I have around 1,200 file to convert. Is there anyway MP3TAG can do the simple change from a to b in the extension ?

Make a new action

Action type: Replace with regular expressions
Regular expression: .m4a$
Replace matches with: .m4b

[ ] case-sensitive comparison

Thanks Dano, Worked fine...