apply for a vew visa

Hello, Im from Chile, and i will travel to Sri Lanka On Friday. I actually have an entry permit for China that expires on August 24th, but I will not use it, because my trip to China was delayed to September. Can I apply for a new one before August 24th having this one still available? Or I can apply for it after this date?
Please let me know because I need to know if I go directly to the embassy on Friday or I have to wait until this date. Also need to know if I need any special thing as a Chilean for this visa.
Thanks!
Francisca