No there is no truth to that at all. All JD 318s from beginning to end of their production run were H2 hydraulics. Some may have been converted by their owners, and Deere did make a flow diverter kit, that through the opening or closing a valve, would redirect hydraulic flow.
Thanks guys, I'm not looking to do it, just was a bit shocked as the guy was super confident in what he was saying. I was pretty sure that the only GT that had H3 was the 140H3 and one of the 400 series i believe. Thanks again.