Is this business a registered "entity" of some sort (something similar to a LLC)?  If not, a Partnership can't elect to be taxed as anything other than a Partnership.   If the business wants to be taxed as a S-corporation, they need to form an "entity", such a corporation or LLC (or some states have other options, another post today was discussing a registered "Professional Association" with the State).

If they do form a brand-new "entity", yes, they will need a new EIN.