As you know this is not an easy thing to do. But I think this is how I would handle it.
1. Start with your ref
2. Cut and paste the things you want changed. For example cut the head, paste it back in place and then rotate it to the position you want it to be.
3. Google facial expressions
4. Keeping in mind the skin tone and features of the original ref then draw in the changes you want to make
5. Carefully paint in the merged areas.
6. Once you have the ref the way you want it to look then start your painting from scatch.
This way you will have an original painting from the reference with a whole new expression or pose. Good luck!
Hope this helps,
"Creativity emerges only when the imagination is given the freedom it deserves."