What will happen is that part 1 and 2 will be completely crosswise, so part 1 will be exactly the same as part 2 from a technical (PvP) point of view. It’s the 1,000th modification on the list that will now coincide with the release of OW2. The biggest change is from 6v6 to 5v5. So the game client is the same, which is not surprising.
But OW1 doesn’t go away completely though, on that OW1 bike as we know it’s already gone a few times. After all, certain heroes have been handled quite a bit over the course of time. For example, Symmetra, Torbjörn, Mercy, Reaper, and D.Va play very differently than the Overwatch version. The maps have also been modified in the meantime, and the formation of the teams has also been changed several times. Overwatch is always going to change (it’s still changing for now, to keep the balance in order), it’s a straightforward service game, just like Fortnite and Rainbow Six Siege.
I also expect that some systems will simply keep the old graphic settings, such as the Switch and possibly also the PS4 and Xbox One.