Your ticket is going to be very very expensive if you do that.

I believe your current fare is: RMB 8224 incl tax right?

If you start in HKG and return to PVG, your new fare will be RMB 39, 305 !!! Because you're basically buying 2 oneways as your starting point and return is not the same.

It was very irresponsible of EK to just tell you to show up at HKG airport ... but of course !

If you really must start from HKG, then you'd have to refund your existing ticket (It is refundable - but usually with a fee of RMB 500 involved), and buy a new one starting and ending in HKG for around RMB 8050. Then buy a seperate ticket back to PVG.