With a single entry visa, can I travel outside and return?

I am going to visit my parents who live in Shanghai for a while. I have applied for a single entry permit. And my flight is booked for early November. They now have decided to visit home (Norway) during the Christmas holidays. This means that I will also go with them to Norway for a couple of weeks. I wanted to change the application to multiple entry, but the embassy told me that I cannot change it once it has been applied. They also told me that I should handle it once I arrive in China. Does anyone know what needs to be done to change the entry permit to multiple entry? Or do I simply have to apply for a new one?