Most of Season 1 happened as we observed last year. The biggest change was that
Hartley Rathaway never attacked the Cleveland Dam and instead started working with Team Flash after saving Cisco and Caitlyn from a time wraith
We know that several things did still occur though:
The final confrontation in Season 1 plays out in a very similar way. During the one on one in the time vault, Barry gives Eobard a disjointed version of events to come. This results in Eobard thinking he makes it back home.
Eobard: I haven't beaten you. If you're still alive... that means my plan fails. And if my plan fails, I don't get to go home, and if that's the case, well, then... [begins vibrating hand]
Barry: No, no, no! Hey, hey, hey! It's the opposite! It's the opposite! It... you trick me. You harness my speed. We turn on the accelerator to create a path for you to go home. I go back in time. You go back to yours. You won.
Eobard: Then why are you here?
Barry: Because when I got back, a singularity had formed. And now the only way for me to learn how to get faster and stop the singularity from happening was to come here. Now.
Eobard: Well, that's good to know.
This is a combination of what Earth-2 Wells does with regard to the Zoom plot line. He
steals some of Barry's speed
This also references of the younger version of Eobard we meet in Season 2. The Wells/Eobard that Barry is speaking with should remember this, but Barry is cryptic enough that it may not seem like the same incident:
Flash and Reverse Flash run through the pipeline until Flash tosses Thawne through portal back to his own time.
Finally, Barry mentions that he goes back in time. He has done this multiple times, both intentionally and unintentionally:
The first time prevents a tidal wave from hitting Central City (this was accidental)
The second is the trip back to watch his mother die (this was intentional) - the singularity also forms shortly after this trip
The third is when team Arrow helps fight Vandal Savage and the loss destroys the city (this was accidental)
The fourth is the current time travel trip he's on (this was intentional)
We know that Eddie still dies based on the final scene in this episode. We also know that the death is in the same fashion as before. Eddie's death "erases" Eobard's existance.
Hartley: [says something in Latin]
Caitlyn: How many times do we have to tell you, Hartley? Ever since Evil Wells got erased from existence, no one speaks Latin around here anymore.
Then even later:
Iris: It's been almost a year already. Can you believe that?
Barry: That's the thing, Iris. Everything keeps moving forward. Question is, when will you?
Iris: I don't know, Barry.
Firestorm and Earth 2 and Zoom
These three plot lines are all linked together. We know that Earth 2 still exists and that Barry has interacted with it. After returning to his time, Barry talks with Earth 2 Wells:
Wells: Yeah, I'm okay. Although it turns out it's easier to find a two-ton gorilla on this earth than a teenage daughter. I don't know what I've done.
Barry: You did what you thought you had to do. You risked your life to save her. You brought her here to protect her from Zoom. You acted like a father.
This confirms that Earth 2 and Zoom still exist. Firestorm is linked to this plot line because
Ronnie's sacrifice is what destroyed the singularity.
Without that action, the singularity would have destroyed Central City.
So, what did change? This adventure did change some aspects of the time line
After returning to the present and encountering the time wraith there
Cisco's weapon is ineffective. It is Hartley's gloves that save Barry.
We then learn that this isn't a one time deal and that he's reconciled with his parents.
Hartley: Well, as always, it's been a pleasure working with you all on this little project. If you need anything in the future, call me. I'm off to a late dinner.
Cisco: Mm, dinner. Anyone special?
Hartley: Very. My parents.
He is also aware of the Flash's identity (though, to be fair...many people on the show are).
Cisco and Caitlyn spent a year working on a weapon to fight the time wraith.
There appears to be two distinct ways of time traveling on The Flash. The first is accidental and when this occurs, he merges with his past self. This results in only a single "Flash" at that point in time. The other is intentional and this results in two Flashes that can (and do) interact with one another. In this instance, it was intentional and the two Flashes interact on multiple occassions. The result of this is that one version of the Flash remembers a time when
Cleveland Dam was attacked
and the other remembers
Being attacked by a future version of himself and then working with that version. This version also remembers Hartley as an ally
How this plays out is yet to be seen, but there are two distinct timelines with regard to this aspect of the Flash's past year.