When they did this with the inter continent wars, they put a cap on how many people can participate. Not sure how if they did do this because people can still sign up to my knowledge. They also made a new client in that instance which I doubt will happen. I suppose when the server's full, the server's full. There just going to add it to the server list is my guess. This will probabbly cause massive amounts of lag unless they find a way to put a cap on it or split servers into groups based upon which server you signed up from.
I can be pretty sure the map is going to be exactly the same, maybe with eslant not accessible.