It needs to be extremely perfect with complete attention to detail. This is for a client who needs to change the photo shoot to another model's face as requested by the ad agency but the model is not in the country anymore to do another shoot. All legal permissions obtained from both the models. Please post here with your experience. It really needs to be virtually undetectable that this is a morphed photograph.
I did such things in the past. How good it can be done depend from photos which need to be morphed. It will be easy if it's photo with blank background but harder if background is complicated. Please PM with details.