I am going to quote myself (lol) from my patreon post from last month:
Sooo, originally, I wanted to release chapter 5 first, and then do the revisions, aka Arc I release, but in of the last posts, I've mentioned that it will be probably incorporated in chapter 5 directly (so no Arc I release). I can confirm this now and I can tell you a little more about already made as well as planned changes.
The biggest one will be a few new scenes inserted into the story. The goal is not to alter the story in any major way, in fact, you don't need to replay the older chapters at all and your saves should be (almost) fully compatible as well. Almost because I've added "Codex" (a glossary) to the game and entries are unlocked as you progress through the game, so with older save you won't have unlocked old entries.
Some of the existing scenes will be modified a little in various ways, there will be new images and/or animations and I try to improve some of the images. Unfortunately, especially some of the renders from older chapters are really grainy and I've decided not to re-render them all, as there is a lot of them and it would cost a lot of time, so it's not in the scope of this upcoming release.