They don't need to do it for Episode VIII as Carrie Fisher managed to finish all of her scenes before she passed away. Whether Leia will appear in Episode IX on the other hand is a whole different question.

Pros and Cons of using CG faces on body doubles vs whole new actors/actresses:

PRO
* The face will look far more accurate to the person's actual face than any other actor will. No matter how much make-up and styling they do to Ewan MacGregor, he will never look like Alec Guinness - he's not a Doppelgänger!

CONS
* Cost. CG animation is expensive, and the level of CG required to try and make a convincing looking human face is extremely costly. If Leia is going to make a brief cameo appearance or limited appearances in Episode IX, then the cost may be justifiable, but if she's expected to have a prominent appearance (as she is said to have in Ep VIII), then the expense may not be worth it.
* CG human faces are never fully convincing to audiences. And this isn't because the CG is necessarily bad (despite what people say), but because of psychology. The human brain is super-duper retentively and pedantically nitpicky when it comes to the human face. This is because paying super attention to the face is an important and constant skill in human interaction, not just for facial recognition, but also in gauging non-verbal social cues. A CG Death Star can fool our brains into thinking that it looks the same as the old physical model from A New Hope because we've never seen a Death Star in real life. Our brains and constantly scanning images of Death Stars for any changes or anomalies, whereas they do with faces. It's constantly automatic and subconscious, you can't stop yourself from doing it. But some fans will still whinge and whinge about how "terrible" and unconvincing the CGI is... *sigh* It's not even a CGI issue. If you look at older films where they used fake heads/faces that were physically constructed, they still don't look fully convincing - and I'd say they look far worse than CG constructed faces today (but were still good for its time).
e.g.
T100 in Terminator
dismembered Alex Murphy from Robocop
dismembered Robocop from Robocop 2
If you look at this image from the ending of "The Day of the Doctor" which shows every Doctor standing together, only the middle 3 Doctors are real actors (and the eyes of the 12 Doctor which a fan has imposed in the background). The rest of the Doctors are wax statues from Madame Tussauds. They look really unconvincing to me - I initially thought that they were CGI. So yeah, the issue isn't so much to do with CG but more to do with just how nigh-impossible it is to deceive the human brain into looking at an artificial human face and believing that it's real.

Here are some images of the most human-like robots that have been made to date next to the real humans that they were modelled off.
https://cdn4.dogonews.com/images/428...84c-medium.jpg
http://media.techeblog.com/images/roboticclone.jpg
http://d.ibtimes.co.uk/en/full/13856...ch-machine.jpg
In a sense, artificial replicas of humans need to pass a sort of visual Turing Test, and we haven't been able to successfully build one convincing enough to do that under scrutiny. Brandon Lee's CG face in The Crow was convincing because the character was shrouded in darkness, and that was only used briefly (as Lee had managed to finish most of his work on the film before he passed away). Here is the puppet used by ILM for Luke Skywalker on Speeder Bike. Up close it looks totally fake, but in the actual shot it was used in high speed distance shots, whereas closer shots where Luke and Leia's faces were more visible were done by shooting the actors on blue screen.