You should do a cost comparison before you take out the furnace.
One Therm of gas (100,000 BTU/Therm) has the same energy as 29.3 kWh of electricity. Because electricity is 100% efficient compared to the furnace, you should compare the cost of 24 kWh of electricity with one Therm of gas.
So if you convert from gas to electric heat, your $400/month gas heating bill will become approximately:
$400 x 24 x ($per kWh) / ($per Therm) = Electric heating bill
That will be in addition to your current electric bill for lights and other uses.
Old houses usually don't have the amount of insulation that is required to make electric heat practical. Most people would give a lot to convert FROM electric heat to gas. There are a lot of people around where I live who put in wood stoves and carry in wood and haul out ashes every day because electric heat is so expensive. Don't be in a hurry to tear out those ducts.