Possible to change return flight? - 72-hour visa free

I passed the border this morning in Beijing, with my return ticket for tomorrow. However I would like to change it for the day after. My stay is still going to be less than 72 hours, but I am wondering if immigration will find it's a problem that I changed my return flight, and therefore the time and flight number are different?

I called the immigration number for Bejing, but of course no one speaks English...