Reconfiguration of the cabins in an aircraft is amazingly expensive, its rarely undertaken unless there is a wholesale refurbishment of the interiors. Remember that any change to an aircraft has considerable compliance requirements and not only oversight from the manufacturer but also agreement from regulators and also the lessor if you don't own it outright.

If business class is full and the airline feels the need to manage demand and capacity then the easy option is to charge more per seat. A win for the airline's revenue without any additional capital outlay.