The changes in the wizarding world took years to implement. The review process, even with extensive help from muggles and wizards alike was thorough and exhausting. Harry Potter had everything gone through with a fine-tooth comb. Some places he changed a great many things, some places he changed only a few. Muggle rights in wizarding courts were affirmed, laws for trials were stricter, more aimed at finding the truth and serving justice than even muggle measures. Too many changes were made to name them all, but it caused much commotion among those entrenched in their ways.
There were always witches and wizards protesting new laws or measures, but with the power of Merlin's Staff backing up the laws they could not change the tide and had to learn how to deal with it. In the first year alone after his reemergence there were three assassination attempts on Harry's life. However, there were five Death Eaters brought to justice which Harry counted as a success, despite the opposing views.
When new generations came along and grew up with the measures it became suddenly a lot easier to deal with. Amusinly, many of the younger generations looked at their elders like they were utterly insane when they learned about some of the old laws and practices.
Ultimately closer ties with muggles were asserted. The wizarding world was still hidden of course, but they were allied with their respective countries and both sides benefited greatly from it.
On the grand scale, the world had changed because of one man's actions.
Dobby became an immense help to the Barnes family as he kept all of David's scheduling coordinated (he continued working at the hospital but had biweekly meetings to keep tabs on things), sort and answer his mail, and became very good at dissecting various bits of information to tell David what was most important. He also took care of Michael while David worked, though David made sure he was a more competent guardian than he had attempted to be when Harry was a teenager.
Amelia's family eagerly gave up their parental rights to David, all the while grumbling about freaks, who couldn't do things the right way when they first indicated they wanted nothing to do with her, and she was quickly integrated into their family. David's actions and changes had gotten her interested in government and so after graduating from Hogwarts entered the Muggle relations department, which was a far cry from what it had been not long past. Muggle Relations was now considered one of the most difficult departments to succeed in, but those in control of the department were those who worked more directly with Harry Potter than anyone else.
Courtney went to a muggle university after graduating from Hogwarts and received her medical degree and then completed an apprenticeship at St. Mungo's. Following in her father's footsteps she became accomplished at combining magical and muggle healing techniques to achieve results not possible with either discipline alone. Her ground-breaking work established Combination Healing as a valid field, rather than the dabbling her father had done.
Michael was accepted into Hogwarts when he was eleven years old and placed in Hufflepuff, which Harry thought suited him. He barely remembered his mother, which Harry found immensely sad recollecting what his life had been like having no memory of his parents so Harry provided him with a Pensieve to put his own memories of her into and ask others for memories as well. This allowed him to see his mother all over again, whenever he wanted, something Harry had wished someone had been wise enough to do for him.
Albus Dumbledore, did indeed retire from Hogwarts at the end of the 2021-2022 school year, but lived to the ripe old age of 210. By the time he died Harry was able to regard him with some fondness again. Albus had had the wisdom to make a completely new relationship with Harry rather than trying to build on the crumbled and broken remains of what had been there before.
Harry eventually confirmed all that had been said about Ron and decided to give his old friend a second chance as he often found himself quite lonely. He could never be comfortable with Hermione, but he and Ron, through decades of work, had a stronger friendship than they had ever had while they attended Hogwarts. Hermione meanwhile, did manage to push past what had happened and became a crusader in her own right for different causes—some diametrically opposed to Harry's measures, some right in line with them. More importantly, she gained a stronger relationship with her younger sister.
Harry had little to do with Sirius at all. There had been too much to overcome. However, when Sirius died at 115, Harry attended the funeral and grieved for what should have been in their relationship.
Harry himself, despite all declarations to the contrary, eventually rejoined the wizarding world, but as a private citizen rather than ultimate leader. He had run into the problem all wizards in the muggle world found—slowed aging. He found himself nearly 90 years old and looking only 60 that and finally conceded he could not continue to live as David Barnes in the muggle world. That was when he found himself most grateful for Ron's friendship; without his co-workers, acquaintances, and friends in the muggle world, he was more alone than ever. The truth was eventually revealed to the wizarding public, that Courtney Barnes' father (who many remembered for his audacious disruption of a Wizengamot trial) was actually Harry potter. By that time though all his children were grown and able to handle whatever came as a result of that. He never remarried, though he did date occasionally.
When Harry potter died at the age of 173 he was mourned, not for his reputation, but for his accomplishments. He was one of the most loved and respected figures in the wizarding world and was put into textbooks alongside such great figures as Merlin, the founders of Hogwarts, and others—but he was counted greatest of them all. In a final letter to the wizarding world a few days before he died, surrounded by his children and grandchildren, and great-grandchildren, he said this:
To Whom it May Concern:
I am Harry Potter, the wizard you betrayed, but it has taken me a long time to realize that I was a traitor too. By hiding, by running, I betrayed my parents and my heritage as well as the future. Thank Merlin I finally saw the truth. This world is larger than me, it is larger than you, it is more than just the present it is the past and future as well.
Pardon me for sounding didactic, but I am 173 years old so I can take certain privileges. Running, hiding, hate, bitterness—none of them are worth the energy and time expended. I prefer to face things head on now. Believe it or not, I forgive you. All of you. I did a long time ago in fact. There were some things that could not be brushed away, but I no longer hated you for them.
Take care of this world. Be vigilant. Be active. I beg you, don't let all that you have accomplished be in vain. Learn from the past, remember your mistakes. But create your future as well, don't just let the future come—meet it. I must be getting sentimental in my old age, that almost sounded wise, if a bit corny.
As my final act I place with you a trust. Upon my death the rights of Merlin's Staff will be abolished. I declare it: so it is. No one should have that much power. If you continue in the way you have been, there should never be any reason for such a position to exist.
Be well my friends, remember yourselves.
Harry James Potter