The cord just plugs into the side of the machine, none of the 400 or 500 series were hardwired into the cabinets. If they have a knee control it is because the foot control is inside a bracket inside the cabinet. A tug upwards and it will come loose. The foot control doesn't have to be wired into the case or connected to the machine until ready to use.
To remove the machine from the cabinet tilt the machine the whole way back and look under the rear edge of the bed where the hinges go into it. You will see a small set screw holding the machine to the hinge pin. Loosen them a couple turns, there is no reason to completely remove them, While the machine is still tilted lift it off the pins. If it won't come off loosen the set screws a bit more.
I hate to see the hinges removed from cabinets unnecessarily because they do get lost and the cabinets are so old there is a real chance of stripping the screw hole in the wood.