The biggest change, and as a Halo fan, the biggest turd in the punch bowl, is the fact that the Master Chief's memories have been wiped.
The Master Chief doesn't remember his childhood. In fact, the conflict between him and the UNSC kicks off when he touches a Forerunner Macguffin and it restores his memories. He reports seeing visions of a white dog and a family back to the UNSC, and the encounter leaves him moody and uncertain. Parangosky and Halsey panic, because they realize that his induced amnesia is failing. His memories of his childhood are coming back. They are terrified that their control over their pet supersoldier is going to slip, and Halsey explicitly proposes restarting the Cortana program to bring the Chief back under control.
If this leaves a bad taste in your mouth, let's compare this to the source material.
The amnesia is actually a plot point from the very first book, the book that established what the Spartan II program even was. When the six-year-old Spartan II candidates were brought to Reach, Doctor Halsey personally briefed them on what would be done to them. Her AI assistant, Kamiya, proposed wiping the children's memories with a selective neural paralytic, and telling the children that their parents were killed in a bombing.
Doctor Halsey rejected this option out of hand, explicitly because the neural paralytic could fail, and then the children would realized that they'd been lied to. Instead, she used the truth. She told the children that they would be conditioned, and trained to be the best that they could be. They would become supersoldiers to protect Humanity.
And all the little Spartans signed right up. John 117 was offered the chance to be Superman, and he took it.
If this leaves you conflicted, it should. Now let's talk about narrative and The Message.
This is just some comedian from Polygon reading through the Halo novels back when there were less than twenty of them, and it's a fairly unremarkable video except for his review of The Fall of Reach. He's shocked that the Spartans are child soldiers, and says that based on the way the book is written, you'd expect the UNSC to be the bad guys. And if Halo was a lesser series, the UNSC would be. If the Halo books were just another dystopian YA series, then the Master Chief's character arc would involve realizing he'd been lied to and turning on the UNSC, perhaps even defecting to the Resistance Insurrectionists.
But Halo is not that story. In fact, the UNSC are the good guys, and if you only played the games, the UNSC seems like stand-up guys. Captain Keyes and Miranda Keys are solid commanders, as are Lord Hood and Colonel Holland. And Johnson isn't bitching about the UNSC forcing him to fight. The focus of the narrative isn't on whether the UNSC is evil, it's a conflict between Humans and aliens.
See, you can have a good debate about how evil the UNSC is, whether the Spartan II program was justified, whether the Insurrectionists are legitimate in seeking secession as a solution to their problems with the UNSC. And it's pretty damn clear that the characters themselves are conflicted. Halsey, whose password to terminate her AI assistant is "Whateverittakes", is wracked by guilt that everything she sacrificed was for nothing because the Covenant were going to win the war. Parangosky is very clearly horrified by the concept of child soldiers. Ackerson would rather be slogging through the mud with a rifle than doing this cloak and dagger shit, but he can't quit ONI because the job needs to be done and he's too damn good at it.
The problem is that there's a lot of writers who can't handle that kind of complexity. They can't grok that a government is a mass of people, each with their own agendas and ideals, often with conflicting principles, and they make the best decisions they can. The line between good and evil runs through the heart of every Human, and you can have good people and evil people working towards the same vague purpose.
To a certain class of writer, this kind of government is literally unimaginable. The government is either hierarchical and oppressive and full of people who are out for themselves (Or at least willing to casually murder bystanders for the sake of the greater good) or it's a communist utopia where everybody is equal and free to do as they please and nobody goes hungry.
The UNSC is in conflict with dirt-poor Insurrectionists and trains children into modern-day janissaries, so obviously it's not the communist utopia. Therefore, in the minds of these writers, the UNSC is fash.
It's perfectly possible to be critical of the UNSC. Personally, I think Halsey's recruitment of the Spartan II candidates (And Kurt's recruitment of the IIIs) was just one small step above child grooming. You could argue that the kiddies consented to be made into supersoldiers, but that's the exact argument that child molesters make.
But these writers don't make a good-faith attempt to engage with the source material, Troy Denning excluded. They warp the facts and make shit up so they can cram the UNSC into the "Goose-stepping Nazis!" box.
The first to do this was Karen Traviss, in her Kilo 5 trilogy. Remember how Halsey refused to lie to her Spartans. Nah, after a week of one of the candidates begging to go back home, Halsey broke down and told her that her parents were dead. And then the three novels took every opportunity they could get to shit on Halsey.
But the UNSC... well, the UNSC is no longer the stand-up organization we know from the games. Instead, ONI is supplying weapons to the Arbiter's enemies in the hopes that they'll all kill themselves. And they're strapping bombs to prisoners and testing out gene-modded grain that will destroy the Elites' food supplies.
Then came the opening to Halo 4 and the entirety of Halo 5, where ONI is obsessing over whether the Spartans have been traumatized by their training, and simultaneously worried that the Spartans might be out of control.
So. Now we're at the TV show, where The Message is fully evolved. The UNSC is an oppressive militaristic government, and only people who work for it are
A) Evil, power-hungry bureaucrats who are obsessed with controlling others
B) Not necessarily evil, but willing to shrug off liquidation of dissident prisoners with "Just following orders" and "For the greater good."
C) Brainwashed and unaware of how evil the government is.
And the only way out is to be made aware of how evil the UNSC is. In other words... to become woke.
Is this a lot to judge from the first episode. Perhaps. But there were a lot of clues laid down in that first episode alone, and I'm willing to stand by my reading. Halo has gone woke, and everything woke turns to shit.