I imagine the transfers will be staggered to avoid massive congestion, and that the highest population servers will be the first to be offered transfers away to lower pop ones. This is pure speculation based on watching the way other games have handled transfers over the years in similar situations though. My only concern is that they may forbid Legacy players to transfer to non-Legacy servers at least for a couple of months in order to minimize their impact on the economies. There's already a gil-cap for transfers I believe.