they will drop but not by much, because most people wont be aware of this news.
Of course once this new model is on the market they may drop like a stone depending on how big the differences are.
This is not easy to predict what will happen, if sony are changing the gpu, then there is going to be a PR hit its inevitable, the only question is how big that hit will be and if its something they can absorb in the longer run in the PS4's success.
If this was something that only affected the VR games would I be bothered? personally no, although I would have sympathy for those who brought a ps4 for VR purposes. The way to go is a bolt on addon, but ironically the media disagree with me there and are suggesting thats the worst thing to do.
I also wouldnt have an issue if the only change made to games was resolution so e.g. 1080p on ps 4.5 and 900p on ps4 providing draw distance, performance etc was the same.