To be clear, the "whole new experience... for players who have repeated Act 1 over and over again for hundreds of hours," is going to be for Full Release, not Patch 9. And as @Rag said, we knew this a while ago, although it's good to hear it repeated again.
Acts 2 and 3 of the story are being actively playtested to ensure they’re up to the same level of polish and you’ll soon discover that there’s much more in the works (or already complete) than what you’ll encounter in an Early Access playthrough. Our goal is for even the players who have repeated Act 1 over and over again for hundreds of hours to feel like there’s a whole new experience in store for them at launch.
The only hints to what will be contained in Patch 9 is the line "In the Community Update accompanying that Panel From Hell, we’ll go into detail about some of the most discussed topics in the community." This implies that some topic of much discussion (toilet chain, reactions, spell scrolls, surfaces, day/night, long resting, level 5, etc) might be reworked in Patch 9. However, it's also possible that they'll only talk
about these topics, and Patch 9 won't contain any of these reworks. Larian might double-down on their current implementations of these mechanics, explaining in the PfH that they tested other implementations but nothing matched with their vision for BG3.