Question: [David or Walter] (Spoiler)

Plot twists in the predictable plot twists ? :O

...I'm kidding, I think that he changed the processor or something like that. I mean he killed Walter, changed the processor's, and so, he got an update (Walter is a better version than David)

The two androids both showcased their ability to move extremely fast so it wasn't a surprise to me how quickly David managed to take the appearance of Walter with the chopped hand included, and voice imitation shouldn't be a problem for an android. 

Well its called a Oversight.... these happen and well with Ridley Scott we have seen a few over the years...

Ultimately we dont know what happened 100% As it happens off screen.. but there are only TWO options.

1) David simply Changes Clothes, and then has enough time to do this, and add the Scars, and take the Hand Off.   The Problems with this... are the hole in his chin, i dont think i remember it being there latter, and the voice box went a bit funny after the fight but it was only minor and so could have fixed itself.

2) David Uploads his Mind into Walters Body, again how would he do this? How long would it take to transfer all those Emotions. The Problem with this is this Walter does not remember Daniels mentioning a Cabin on the Lake.... unless when he Transfers his Mind/Memories over he Erases everything of Walters and so he has no knowledge of any Memories Walter has.

What if Walter stayed on the planet and helped David? That could be a possibility as well. Remember Walter had the knife and David asked him a question and David did seem to be able to influence Walter a bit. There are a few possibilities.

He is very fast, he is very intelligent, and he has advanced futuristic tools to aid him.