It was never a graphics *upgrade*. It is a compatibility update for xbox and ps3 performance for the changes they want to make to environmental lighting and shadows to make the game look brighter and more lighthearted in the new expansion after how intense Endwalker was.
Graphics on all platforms is limited to what the oldest console the game is available on can handle.
Due to the desired changes in environmental lighting and shaders, they have to change or get rid of some detail shaders that appear on characters and even go so far as to change models slightly. Characters are also now a light source themselves.