I buy a 8700g but the pin this being already used by the previous user registration the PIN, I want to know if I can change the PIN

You cannot change pin, but you can contact your wireless provider for assistance with that, they would have to remove your bb from the previous owners account.
Or you can try and contact the previous owner and ask him to remove the bb from his account.

This happen someone I know. The best thing to do is to return the bb and get another one. It's gonna be very hard to make the change. Return it as soon as possible.