Pre-purchased tickets

Hello,

I have already bought tickets from MAN - HKG.

If I changed my flights MAN - Beijing PEK instead, then fly from PEK to HKG will I be eligible for the 72 hour visa free?

If so and I were to purchase a flight from MAN - PEK and landed on 11/03/2018 at 7AM what day would I have to catch a flight to HKG by?

Thanks in advance