There's no doubt that the Nikon D850 has photographers excited — a lot of them. The demand for the camera is skyrocketing, so much so that Nikon has already issued a statement that it may not be able to fill all preorders in the first batch.
As the D850's release date of September 8 draws nigh, preorders are flooding in at such a pace that Nikon recently sent out an email notifying preorder customers that some may not receive their camera on the release day. While such an event is not unheard of, considering the camera was announced less than a week ago, it's indicative of the extreme demand for it. It's not surprising considering the camera is probably the most well-rounded ever released and early first looks have been positive. This likely means that any preorders placed henceforth will be subject to a delivery delay. Still, orders will likely be filled in the order they were received, so if you do plan on ordering your own, do it now.
Nonetheless, if you were planning on selling your D810 in anticipation of receiving the D850, you might want to hang on to it just a little while longer. We're certainly looking forward to trying out the D850 ourselves!