Would it be more cost effective to retrofit old petrol cars into hybrid or electric than building new electric and hybrid cars?

If you take all costs into account i.e. materials to build car, resource extraction costs, full lifecycle from initial material extraction to sale point etc., given that old cars are already built and all they need is changing the fuel system to either electric or a hybrid engine, if governments were to provide similar subsidizing to this type of conversion as they do to new EV/Hybrid vehicle purchases, would it actually be cheaper to retrofit old cars with new engines?