There are various levels of that. You could have things damn near from scratch. You could reuse the same game engine (like Unreal). You could reuse more, where

There are various levels of that. You could have things damn near from scratch. You could reuse the same game engine (like Unreal). You could reuse more, where it’s mostly the same game code with new content. Civ 5 -> Alpha Centauri was like that. Then you can have expansions or DLCs that really are just additions. I imagine (hope) this change is on the deeper side. GTA V is quite old.