The stock must be exchanged with the transfer agent and broker accounts updated. I have Scott and I will be dead in the water for about a week until they get the change done. Just went through one a couple weeks ago. Some brokers are on top of things and the symbol gets changed right away and you cna trade it the day the symbol changes.
Most brokers will advise to hold until the change is actually completed before selling though, as just in case insurance.
Scottrade will let you sell it if you want(not that you would want to) before it appears in your account ( 3 days last time), but they hold you responsible if the change does not go through. So in essence they are shorting the new symbol for you and you cover it when the new shares arrive in your acct. Had to do it over the phone though.