Is it necessary to replace a furnace when replacing the air conditioner?
Bob, the coil unit (evaporator) in your air handling section is the only part that needs to be changed. With new units this may mean a new sheet metal frame to house the new evaporator coil but I find many furnaces out last at least 1-2 AC units. 

Don't let them talk you into cleaning and reusing the coil or line set if it is a R12/R22 refrigerant type as it's almost impossible to get all the old oil out and then your new unit will have a much shorter life.