How to replace sad face with happy face
Digital manipulation might still be a good starting point. 

If you have the digital film footage from each of these episodes, and if you have good video player software, you should be able to move frame by frame through all the views of her face until you find a smiling one where her face is at the same angle and elevation as in the sad picture.

You can then capture that screen and digitally cut her face out from it.  Try to place most of your cutout lines midway across zones of relatively featureless, flat skin to make the next "merging" step easier.

Once you have her happy face floating in an otherwise transparent image, paste it into a copy of the sad picture, scaling it so that it overlaps and thus replaces the unhappy facial features.  You don't have to get the edges perfect.  Focus instead on placing the eyes correctly and getting the scale right.  

You aren't trying to produce a refined piece of digital art; just a crude template to guide your manual painting efforts.

Once you are happy with the digital results, you can print it out or paint from the screen image or even use a digital projector to cast the image onto your canvas for tracing.