Locking period expiring before closing

I have locked my rate but it expires 9 days before the closing. What are the standard rules for these kinds of conditions? I have heard that banks can extend my locked rate with or without charges given that the rates won't drastically go up. But I am not sure if my perception is correct and also what would happen if the rates go down (meaning that the extension of the rate is bad for me because the rates will be lower). Please give me some suggestion for a normal practice. Would negotiating with the seller to close earlier but staying in the apt to the date of the closing be an option (the seller is not planning to close earlier so that is not planning to move out earlier than the closing date)?