In past years, most events submitted before the regular deadline (March 13 this year, I think) have made it into the catalog by registration in May. But, as others have stated, they will continue to add events pretty much up to the week before the convention. You can then keep changing your events, dropping some and adding new ones, until the convention, with one major caveat.
For events that still have paper tickets, once those start printing (mid June), you will no longer be able to drop those events and replace them. If you find something you like better in the same timeslot, you are basically SOL until Gen Con starts and you can turn in those paper tickets.
Of course, by then, the new events you wanted will be full. ;-(