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After going back and forth with the portrait Harry finally agreed to accept the painting's help with the rituals.
This is not how he expected this late-night trip to Hogwarts to go, he was just there to start making preparations and make sure that he has everything ready for his seventh birthday. This makes it sound like he is preparing for a birthday party rather than the vital life-changing opportunity it really is.
After discussing things with the old portrait Harry made his way out of the ancient castle.
It is a clear night so the moon is perfectly reflected on the lake surface, under the moonlight with the castle in the background and the peaceful lake in front of him it made for a truly beautiful sight.
It reminds harry of the time when he was just a student here, he would often spend his nights looking at the lake from his window when he couldn't sleep. The view is the same but to him it feels like an entire lifetime has passed.
Just like this view this world might look like his old world but that is just the surface, if you look deeper you soon learn that they are two completely different worlds.
As almost a seven-year-old Harry has lots more ways to get information than the time he first whole up. Now after spending hours upon hours in the Potter library, reading old newspapers, subtly getting information from people by pretending to be a curious child, etc he finally has a decent idea of what happened in this timeline and how it differs from the last one.
The changes start a little before he was even born, Death did warn Harry that while he could only go back till the movement of his birth but Death,Time, Fate and others can go even further back.
Obviously these changes are far too numerous and subtle that he would never be able to notice them all. So he only tried to keep track of major changes or the ones that were directly linked to him.
The first major change is the survival of his grandparents. This change occurred because of one man Arcturus Black, older brother of his grandmother Dorea Potter neé Black. So his grandparents and Arcturus were pretty close, they even fought side by side against Grindelwald and his forces during the war. While his grandparents and Black survived the war his wife didn't, after the loss of his wife Arcturus handed control of House Black to his son Orion black and left Britain to live alone.
In the previous timeline, his grandparents were attacked by death eaters in Diagon alley, they were ambushed by more than 30 death eaters. Even at such a huge disadvantage the couple were able to kill 23 death eaters before they finally succumbed to their deaths. There is a reason why these two are counted amongst the war heroes of the Grindelwald era.
In this timeline however Arcturus Black was also there to meet his sister and his old friend, if the death eaters struggled to kill just two people of that calibre then they didn't even stand a chance against three of them. 25 death eaters were killed before the rest were able to escape.
Harry's grandparents were attacked because they opposed Voldemort. After the attack House Potter went on a complete lockdown. This lead to the second major change in the timeline.
Harry was born in a hospital in the previous timeline so there were official records of his birth with the exact date and time. Records which a death eater could easily get with a big enough bribe.
This time however with House Potter taking all the security measures possible harry and his two siblings were born in Potter Manor, under powerful wards and with the help of trusted medi-witch. This time there is no official record of his birth.
This lead to Voldemort attacking the Longbottoms instead on that fateful night. Alice and Frank Longbottom were tortured for hours but still didn't give up their son. While his death eaters tortured the parents Voldemort worked on breaking the wards that protected little Neville.
According to the newspaper after Voldemort managed to break the wards and reach the baby he suddenly disappeared into nothing.
While the death eaters were shocked at the sudden disappearance of their master the Aurors had arrived and captured them. This led to Neville being credited for the death of the Dark Lord and being labelled as the boy-who-lived.
In that same paper, there is also an article about how the potter cottage in Godric's hollow was destroyed on that same night.
With all of this information Harry has a pretty good idea of what happened that night.
When Voldemort saw Neville he somehow realized this is not the baby that the prophecy talked about, he was also aware of the triplets due to Pettigrew and had access to the Potter cottage. So he apparated to Godric's Hollow to kill the triplets.
That same night after harry's parents left the house in hurry after getting the news about the attack on longbottoms, Voldemort came and found only the triplets alone in the house completely defenceless. Just like last time, Voldemort tried to kill Harry but the curse bounced back on him, but this time without his mother's sacrifice ritual interfering with the prophecy Harry didn't get stuck with a piece of Voldemort's soul on his forehead.
These changes are minor compared to the changes caused by his parents surviving that night. Lily and James Potter being alive caused massive changes to the entire wizarding world and not just wizarding Britain.