You don't have to swap anything, the Truma 'Ultra-heat' electric heating is an 'add-on' you screw to the casing behind the gas fire, everything to attach it is already there.
As you already have the fan, this makes the whole job very simple, also you may be interested, I have a complete brand new kit which already includes my upgraded control board and a remote temperature sensor, the sensor being extra in the original kit.