This may be a very expensive process because of the expense of removing the asbestos. Many people use the term furnace to describe both hot water and warm air systems. A furnace is the fire chamber on early boiler systems. When warm air became popular, since the warm air appliance did not have a water vessel on top of it, a warm air unit started to be called a furnace. A water vessel appliance was called a boiler.
Very few warm air (furnace) gravity systems exist today; their chambers burned out a long time ago. If the system truly is a warm air furnace gravity system there is nothing you want in it. The entirety is to be discarded and the heating system installed as a retrofit system. This can be extremely expensive.
A hot water gravity boiler system is another story. Since the pipes are large and the heating units are radiators, much of it can be kept and repaired as needed. The asbestos-covered pipes can be sealed with paints of a safe kind to prevent air-born asbestos. the system can be fitted with non-electric radiator valves to provide comfort levels in individual rooms. The large pipes make the sytem somewhat inefficient. Complete replacement of the entire system does involve disposal of asbestos which can be expensive. A new hot water system sill be a good long-tem investment if you love the home.