system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is actually done to a PS3 game like Uncharted when it's "remastered" for PS4?

Does the entire thing have to be "reprogrammed" from scratch?
gpt
Not typically. More often they'll take the original codebase, update the models with nicer ones, maybe touchup bugs or well-knowni issues, maybe work on the lighting or rendering to make it look nicer, etc. 

It can certainly be a large undertaking, but it's by no means as big of an effort as a full code re-write.