kristarwars said:

They've done it like a million times from way back in the days of The Crow and up through Tron: Legacy: taking a current star (or actor/stunt double/etc.) and superimposing another (and sometimes younger) face on them. Even Episode I had Ewan McGregor and Liam Neeson's faces superimposed over their stunt doubles for a couple of shots. I'm sorry, but they can totally do it.

being able to do it and being able to do it convincingly are two completely different things. The Crow was just a few brief straight on shots. All the head replacements/ digital doubles in the prequels looked terrible. The Jeff Bridges digital younger self still suffered with the "uncanny valley" problem that plagues CG human creations. They still can't do it convincingly to this day.