If it needs a redesign anyways (since it's ancient) then go do it.
Drops will happen. Nobody can tell you exactly what will happen because some experienced big drops. Some, none at all. I've experienced mostly positive with it. Dropping a couple spots but eventually gaining it back
The only thing we know is that there's a lot of work involved.
Just be sure the social branding is changed, make it consistent and be sure the urls are properly directed to the new urls. Let the thin pages die and direct them to a cute 404 page. Be sure to use their list to let their previous clients know.
Just warn them, let them know what's involved, the many hours that theyll need to pay and what CAN happen. Let them sign a contract if needed.
It might even net you some good local PR which will just be great links.