Do I need a transit visa to change flights in Shanghai?

I'm a South African travelling to Hong Kong in August but would be going through Shanghai... on the return trip I need to change airports in China (travel from one to another). While I wouldn't need a visa if I were to stay at one airport do I need one if I'm travelling from PuDong to Hongqiao? I've been told that I might be able to get a permit at the airport in China itself but isn't this risky? I don't really have time to apply for a transit visa.