Apart from the diameter being reduced. The rapid barrels will need the transfer port worked quite a bit, as the BSA transfer port is a totally different design that uses brass bobbin and seals. So the barrel will need a recess for this.
It maybe be easier to cut off the existing transfer port and start fresh again. Also upto about half an inch of the rifling of the barrel will need to be reamed out to accept the BSA probe and seal.

Can tomorrow take a picture of BSA and Rapid together to give you an idea of what will need to be done.

In fairness 24" rapid barrels fetch strong money, you could easily sell just one rapid barrel and still have change from purchasing a barrel blank.