It would be inaccurate to say that Christ existed as a human being prior to the Incarnation. However, there are some interesting thoughts some theologians have spoken about regarding the incarnation as more than just an historical event.
For example, I believe it's St. Irenaeus who said that even had Adam and Eve not fell, the Word would have still become flesh. The Incarnation, therefore, should not be understood simply as a response to sin, but as God's eternal purpose for mankind and creation being located in Christ who became flesh. "All things were created by Him and for Him".
There are various theological strains of thought and discussion regarding the complexity of the notions of Logos Ensarkos/Logos Asarkos, I don't really think I'm well read enough in that area of theology to make too many comments. But from at least some basic idea the issue involves the notion of the Incarnation being an intrinsic part of the Logos/Son; not just as an historical event but as an eternal reality (at least in some sense). Or, conversely, if this is not true. It's one of those areas of theology that, I think, can get exceptionally deep and rather complex.
-CryptoLutheran