Retain:
Good thing about the face you selected is that the lighting and shadows match.
The amount of blur applied on the new face layer is good.
Improvements:
The previous girl's hair still shows where the Red outline markings are. (Sugggest using a new layer on top of the short haired girl's layer and clone stamping the background to eliminate the bleed)
The Green marking shows the head is too oblong without any interfacing between hair and background. By this I mean little tufts of hair getting disheveled and flying out. (Suggest adding a hat / other head covering or make a better selection of the hair)
Again, if you see the hair in the clipping below, you see that the hair does not interact with the background layer that much. There is no definition to the hair - where it parts, or how it flows and there is no lighting on the locks of hair, which is why it does not look real. All the hair in the center is devoid of sunlight and highlights. If she is leaning backwards, there should have been more sun light on her hair.