It did say something about that in the calibre description

but it wasn't clear if that was before or after the regex.

for the renaming -- that would be easier than some of those Windows file mangling tools.

(I hate having to tell Windows users to hunt one of those down and do something I don't know with it.)
But you don't need to exec bash unless you specifically want to override your current shell. Just use:
Code:
find . -name '*_*' -exec mv "$0" "${0/_/--}"