Not replacing both will severely hurt your new air conditioner’s energy efficiency. You see, your air conditioner’s inside unit and your furnace both share the same air blower, meaning you can’t replace the inside air conditioning unit without also replacing the furnace.
So, if you just try to air conditioner, you’ll only be replacing the outside unit, meaning you’ll still have an older inefficient inside unit. This not only keeps your energy bills high, but it can harm your new air conditioner unit. Some manufacturers won’t put a warranty on an inside-only unit because they know the harmful effects of just replacing the outside unit.