Well, yes. Some people may leave, some people may not. There is no controlling things such as that.
However, I myself feel obligated to stay since Last Moon was the first IT server I had played, so I felt I couldn't just leave it unless it was due to something I could not control, you know? I feel some people feel the same way I do for their own reasons. But, as Faint said, there WILL most likely be a slight decrease in players; however, there will more than likely be an equal amount of players. I myself will play both DoE AND LM, even though I will probably play Last Moon more often more than likely.