Similar to what I'm wanting to do with my current bike when my new one is fixed up.
I've got a flat bar, rigid frame Stumpjumper; I'm looking to put a threadless stem converter, a higher rise stem, and some On-One Midge bars with bar-end shifters... Add a few more hand positions and some extra comfort to my commute.
There's no reason why you would have to change out your cassette and derailleurs to go with the setup you're talking about. I think the LBS is just trying to pull a fast one on you. (Too bad for them that all of us here are a little bit faster. )