A system for automatically varying the mold width and thus the paving width of a molded concrete slab on the fly as the slab is being molded. An external stringline reference is used to control the height and direction of a first side of the paving machine. A slope sensor is used for automatic control of the height of the second side of the machine frame. A starting point signal generator provides a signal to initiate the automatic width transition. A controller controls width actuators for controlled variation of the mold width in response to the starting signal and a pre-programmed function of the controller.