This is not true. The oil in the high pressure system is the same oil as the oil in the pan. It's constantly getting exchanged, the low pressure oil pump simply pumps oil from the pan to the high pressure oil reservoir, then to the HPOP, and eventually through the injectors and back to the pan.
When you've done a fresh oil change, it only takes a matter of seconds for the new oil to get pumped from the pan to the top side of the engine to the HPO system. Save yourself the time and energy, it's a totally waste.