The new PVP.net patcher is now live for League of Legends. If you're currently logged into the game, you'll download it automatically the next time you start up the patcher. The initial download is fairly small, but once you've gotten everything migrated over, you'll have a bit to go before you're done.
In all, my download clocked in at about 150MB in size after getting the new patcher up and running. It also performs a full scan of your game to ensure there's no corruption from the switch.
So if you're planning on playing at a specific time tonight with your friends, you may want to start the patch process now so you're good to go later on. It takes upwards of 15 minutes in total for most computers.
If you're already running the new patcher and don't like your download speeds, you can disable peer to peer -- it's enabled by default and sucks up a lot of bandwidth, slowing your download speeds considerably in most cases.
At some point Riot may change it so disabling peer to peer is not advantageous, but at least for now it was for me. Your mileage may vary, but reports I've heard all place peer to peer usage as a bad idea at the moment.
Unfortunately initial reports from the community is that Riot has also disabled the ability to use custom skins on the patcher. So, if you were a fan of using them, you're out of luck -- at least for now.