Changing single entry to double entry chinese visa

We are Australians who already hold a single entry type, we need to cancel it and apply for a double entry one as one of our future flights stops in China twice en route. Can we apply for this new one while we are in Tokyo, what will the cost be, what currency do we pay in, and do we need to go to the embassy or can we do it through a travel agent? We leave for Tokyo tomorrow night and are there for 5 days. Thank you so much.