This information comes from a recent AMA on Reddit. Acting as an interview of sorts, it allows the online community to ask questions towards the host en masse. One of the responding developers included Aaron Keller, the director for Overwatch 2. Reddit user playerDbD asked what early access means for the game and how its release will affect the original game. Keller responded with the following.
Were using the term Early Access to indicate that this is just the start of many new things coming to the game. Were launching with new heroes, maps and features, but there are even more of these coming seasonally every 9 weeks. We recently released a roadmap detailing some of this content, with a new hero coming in Seasons 1 and 2, and a new map in Season 2. Additionally, larger pieces of the game that have always been a part of the vision for OW2 will be released to the game as part of the live service, including the launch of the PvE Campaign next year.
When OW2 launches on Oct 4th it will be a replacement for the current Live Service.
This is only the most recent in confusing phrasing used for Overwatch since the sequels announcement. Previously, former director Jeff Kaplan described both games as existing in a shared multiplayer environment where no one gets left behind. They also stated that progress from the first game will carry over, and that it will be a free-to-play title. It's not clear how exactly it is going to "be a replacement" for the original. The common belief so far is that when it becomes available on game marketplaces, Overwatch 1 will be removed. It's also theorized that it will literally replace Overwatch 1, acting as an update instead of a separate game.
Throw in the knowledge that it will effectively replace the original game, many folks are considering Overwatch 2 to be less of a new game and more of an overdramatic marketing scheme to promote a content update and new monetization method. And to be frank, we sort of agree with the mindset.
So if you find this whole process confusing, think of it like this: Overwatch is getting an update in October that renames it to Overwatch 2, goes free-to-play, and will start getting more content updates in the future.