So when the new client hits will we need to re-download the game?

Most likely what will happen is riot will make it so that either both clients work for like a week or they will just have one day where you only the new one works. After downloading and installing the new one, it will just relocate the files most likely and everything will be fine. Either that or the old client gets updated to be the new one

The patcher will install the new client. All of the in-game stuff won't have changed, it's just the launcher and out-of-game client.