Conversation of S1 to S2 visa

Hi, my daughter who is 18 years old got S1 from my country to come and stay with me in China, but on arrival the PSB people are saying that she could not get S1 and so she has to go back and get S2 and come again as she is more than 18 years of age. I will appreciate if you could tell me that if S1 can be changed into S2 without travelling back? And also I could not understand that why the Chinese embassy issued her S1 visa if it's only for children under 18 years of age?