This may or may not be new information, but I have seen on other threads people saying that the exisiting time table may remain. Just got this reply from a memeber of Southern staff on their customer forum:
May stop people from worrying as much about public transport back.
We have been in discussions with Brighton Football Club, local authorities and emergency services regarding the provision of services before and after each match. As a result of these discussions, we have a number of plans in place to cater for the additional demand which include extra services and strenghening of existing trains. Plans are still being finalised and will be advertised in due course.
May stop people from worrying as much about public transport back.