2. Improve 8th gen games? Very doubtful, as that would need new textures and code on games that don't sell anymore. It's just not worth it for the publishers, they'd rather release a remastered version to sell it again to you. Sony might do that for their own games, but that's about it.
No, it wouldn't need new textures and code.
Thanks to the PS4 Pro and Xbox One X most PS4/XBO-games already use dynamic resolutions. The textures are already made for 4K-resolutions, even if the current console hardware reduces the resolution most of the time.
A game with dynamic resolution in a range of 1440p to 2160p on PS4 Pro / Xbox One X could run in solid 2160p (4K) without changing any code. A game with unlocked FPS in a range of 40 - 60 fps on PS4 Pro / Xbox One X could run with solid 60 fps without changing any code.
Additionally Sony has patented texture replacements for older games: https://www.eurogamer.net/articles/2018-10-07-sony-patent-sparks-playstation-5-backwards-compatibility-rumours-again