Adding open/closed icon to Twitter Bootstrap collapsibles (accordions)

I've set up this simple version of the Bootstrap accordion:

Simple accordion: http://jsfiddle.net/darrenc/cngPS/

Currently the icon only points down, but does anyone know what JS would be needed to be implemented so as to change the class of the icon to:

<i class="icon-chevron-up"></i>

...so that it points up when expanded and toggles back to down again when collapsed, and so fourth?

Hmm...

Thanks for your time! :-)

Darren