I've not replaced a back boom, but I did move the back boom out on the old trike when I fitted the hardshell seat.
How you do it:-
Undo all the cables.
Split the chain.
Take the seat off.
Undo the two allen bolts holding the boom in place.
Slide the boom to its new position.
Lock boom in place, look over the back wheel and up the frame to check its square.
Replace the seat.
Add a bit of chain.
Replace all the cables.
The only difference with replacing the back boom is moving deraillier and brakes over to the new boom.
And you probably don't need to extra chain.