Overwatch 2 will replace the original game of 2016. The developers told about it during the AMA session about the future of the franchise on Reddit.
Q: What does the term "Early Access" means for Overwatch 2? Does this mean that on this day OW1 is getting replaced by OW2's PvP, or will OW1 shut down once the game is out of Early Access?
A: We’re using the term Early Access to indicate that this is just the start of many new things coming to the game. We’re 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.
It contradicts what the former game director Jeff Kaplan promised during Overwatch 2 reveal In November 2019. Both titles were meant to co-exist together and share the playerbase.
Some previous maps like Temple of Anubis and Hanamura or the 6v6 PvP may still keep their place in the custom games. The developers may also rework them to fit other mods, although these maps won't be available at launch. They also confirmed the rumors about social systems, with guilds being their top priority.
It's worth noting that Overwatch 2 will be free-to-play. It is currently running a series of beta tests that is available for everyone who signs for it. They revealed the new tank hero Junker Queen and promised the sequel would introduce Battle Passes and will get rid of loot boxes.