Although I absolutly agree with all previous posters that this is very risky, my single experience is different.

Booked CGQ-PVG-HKG on MU. Couldn't make CGQ-PVG, as I already was in PVG. Called the TA, was informed, PVG-HKG would be cancelled if I don't fly CGQ-PVG.
Made a new booking for PVG-HKG, same flight.

At check-in, the lady asked which booking I wanted to use, as I had two. They did not cancel the original booking, despite I didn't fly the first leg. Took the first booking, cancelled the contingency booking (fully refundable Y). If this is a way to go for you, I'd try it.

Let me say again that the normal procedure is what all other posters have stated.