Action to change extension?

Is there an Action that I can use to change the extension of a file?

When I use iTunes to convert an MP3 audio book it gets generated as a M4A file.

I use M4B for AAC audio books and have to use another program to make mass changes before I use MP3TAG

It'd be more convenient to just use MP3TAG

In the default installation, m4b files are already read by Mp3tag.
If you want to change a filename, you can use the function
Convert>Tag-Tag
for _FILENAME_EXT
enter as format string:
$replace(%_filename_ext%,m4a,m4b)

Use an action "Replace using Regular Expression" ...
Field: _FILENAME
RegExp: ^(.+).m4a$
Replace: $1.m4b

DD.20150305.1940.CET

Thanks to both

Appreciate it