I have bought an l67 motor to put in my vy v6. I have been told that I can just use my old v6 harness and PCM/ECU and have that reprogrammed to suit such that it runs the l67 and works with my standard BCM still. Will this actually work?

It will, however, you would have to slightly modify your existing wiring loom to suit the L67 engine. In saying this though, it would be far easier than having to swap over wiring looms from a VY L67 to suit. Please note that we have a PCM/ECU to suit such a conversion as shown in the drop-down menu from the link below.