This doesn't depend on where you change it, but on which bank you change it. Banks exchange foreign exchange at their own exchange rate every day, and there are often subtle differences.

For example, if you go to Bank of China, the exchange rate of cash is uniform at any branch of Bank of China in China on any day. But if you change it at ICBC, the exchange rate will be 0.0% different.

Upstairs, it is said that only China Bank is engaged in foreign exchange business, which has long been outdated old information. Now any state-owned commercial bank can handle foreign exchange-as long as the outlet you go to has a foreign exchange window.