One of the Old Boys who got me started was mad keen on trams, but didnt have much money, so he converted his LGB fleet when the fancy took him. He mainly ran every Thursday on the living room floor, when his wife went to Bingo, and modelled in card.
He effectively made drop on bodies to fit over existing models, and glued them in place with UHU, which was strong enough to hold the body in place, but could be removed with a bit of acetone. Likewise, if he needed to paint the donor model he would use emulsion paint so it could be scrubbed off. Ironically he never reversed any of his conversions, but after his death his son undid all the work and sold the original models before we had chance to bid.
Anyway, a Charlie Band BiCabine conversion would entail a cab, side tanks and skirts made in one piece, and sitting on the flat plate at the front and the two flat plates above the valve gear. If the cab was wildly different then he would lightly glue on new cab sides.