A method of converting an original FCC unit of side by side configuration to a converted FCC unit for processing light feedstock by replacing an regenerator in the original FCC unit with an embodied regenerator. From the original FCC unit, the air supply assembly is removed and a centerwell is installed with a fluidization gas, fuel, and air inlet(s) through the centerwell. Distribution rings are connected to each of fluidization gas and fuel inlet(s). An internal pipe and a standpipe portion are installed, wherein a lower end of the standpipe extends into the centerwell creating a radial slot, and wherein the lower end of the standpipe is spaced above the deflector plate in the centerwell to allow flow of spent catalyst through the standpipe and provide deflection of the spent catalyst flow for mixing the spent catalyst with fuel oil that is vaporized within the centerwell.