My 18 year old 30 gallon electric water heater (240v/3500w) now plugs into a wall receptacle served by a 20a double breaker. The water heater just gave up the ghost and started leaking. The replacement is a similar sized/similar powered unit but code now specifies that the connection should be hard-wired.

Do I just remove the existing plug cover and receptacle, wire nut the new wires into the box, pass them through a metal face plate into bx joined to the face plate, then to the water heater? The service to the water heater is with #12 wire in rigid conduit to a metal box at the wall.

Is that the proper way to convert this setup or is there some other consideration/material to use that make it a proper connection?