I applied for single entry visa. Now I changed travel plan and I need to enter China twice, is it possible to change the visa to double entry and just pay the extra amount? Can I extend my stay in China while I am there?

China Visa FAQ by FreeChinaVisa.org

Unfortunately, visa once issued can not be changed. You will have to apply for a new visa. You need to go through the whole visa application process and pay the fee for the new visa again.

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