Outfitter should do. You can usually make it so you choose outfits on certain criteria, so set shadow form as your dps weapon and caster form as your normal outfit and you should be fine.
http://wow.curse.com/downloads/details/4784/
Quote:
Outfitter allows you to create multiple outfits and equip them with a click of your mouse. You can layer outfits, create outfits optimized for a particular attribute (ie, fire resist or maximum defense), and have outfits swich automatically equip based on events like mounting up, stealthing or changing stances and forms.
From above link.
edited to add emphasis in quote.
Edited, Apr 17th 2008 1:06am by mundaneboy