https://www.epo.org/en/node/epo-international-searching-1

I have selected the CNIPA as ISA. Is it possible to change and select the EPO as ISA instead?

No. Once you have filed your application and selected the CNIPA as your ISA, it is no longer possible to change. Furthermore, in order to be able to select the EPO as ISA under the pilot, your international application must be filed in English. Applications filed in Chinese will not be accepted for the pilot.

However, if you have selected the CNIPA as your ISA, you may submit a request for a supplementary international search (SIS), choosing the EPO as Supplementary International Searching Authority (SISA). A SIS request must be filed with the International Bureau within 22 months from the priority date. If your application was filed in Chinese, you must also file a English, French or German translation of it with your request. The EPO will establish the Supplementary International Search Report (SISR) within 28 months from the priority date.