There are plenty of stories of the forces of the imperium turning to chaos, being corrupted or just becoming evil. Is there any indication anywhere that the same can happen the other way. Forces of Chaos renouncing the dark powers and fighting against those they previously fought for?
In the Gaunt's Ghosts novel Blood Pact, Mabbon Etogaur is introduced. He was an officer of the Imperial Guard who defected to the forces of chaos and then defected back year later ("Etogaur" is his rank among the forces of Anakwanar Sek and is essentially equivalent to being a general). He is a recurring character in the next few novels, and although he is kept as a prisoner, he provides valuable intelligence that allows the Ghosts to pull of a raid deep in enemy space. During his incarceration, he reads imperial religious pamphlets and seems to befriend some of the soldiers guarding him from assassins.
Sergeant Castus of the Ultramarines
Seraphim Aescarion was the sister who witnessed the fall to Chaos of Sergeant Castus of the Ultramarines 2nd Company. She made it her life's mission to bring this Heretic to justice. She prevailed after many standard decades and returned the Chaos Champion of Nurgle to the fold of the Imperium. She gave him a chance at redemption by sacrificing himself in the service of the Emperor. Castus carried an Exterminatus-grade weapon within his own body which he used to purify the world of Saafir of the Lord of Plagues' taint and send the Daemon Prince Parmenides howling back into the Warp.
It's implied he is under the control of the demon, he's not a bad guy but the demon completely wipes his mind and reprograms him, so the chance for redemption is there once he regains some of his memories.
Scaevolla of the Black Legion (sort of)
In the short story Honor Among Fiends, Scaevolla, who was a Luna Wolf back in the day, is tired of all the killing and being used by Chaos for entertainment so he decides to resist the Dark Gods. Or at least try.
Even then, Scaevolla is angry at Chaos for being used, as opposed to deciding he wants to be a good guy again and join a loyalist legion to fight against Chaos.
Horus felt regret at the very end
At the very end, just as the Emporer completely wiped him from existence, Horus felt remorse for what he had done, he had been tricked after all:
The casual brutality of the Warmaster's act galvanised the Emperor as He realised what awaited Mankind under the rule of Horus and the Chaos Gods. Realising at last that His favoured son was truly lost to the corruption of Chaos, the Emperor finally gathered His full and awesome psychic power in the Immaterium and unleashed a lance of pure Warp energy that pierced the gloating Horus' psychic defences and ripped his body apart. In some versions of the tale, this blast was only able to pierce Horus' body through the hole that had been made by Sanguinius before his death. Just before Horus died, he looked his father in the eye, shedding a single tear, begging his father to forgive him for his betrayal. The Emperor saw regret in his fallen son's eyes. The Emperor also knew that the Ruinous Powers could attempt to possess Horus again, and that He would not be there to stop his son again if they did. Driving all of the near-infinite reserves of compassion from His mind for the sake of the humanity He had served and loved all the years of His long life, the Emperor destroyed Horus utterly, his essence burned from existence in both the physical world and the Immaterium so that the Ruinous Powers could not resurrect Horus as a Daemon Prince through their claim on his soul.
I remember the treatment Uriel Ventris received at the hands of the Grey Knights after he escaped from a Chaos prison. They more or less tortured him to death to make sure there was not a trace of corruption left. So even if you did turn against Chaos, there's nowhere you could go. The Imperium wouldn't want you back, you would be too much of a risk in their eyes.
The Exorcists Space Marine Chapter
From WH40k-Fandom-Wiki: The Exorcists were founded as part of a highly-classified Imperial experiment to create Space Marines who were unusually resistant to daemonic possession and Chaos corruption. As part of their initiation into the Chapter each Exorcist Astartes had actually been forced to serve as a daemonhost for a short time before having the foul creature expelled back to the Warp by the intervention of an Inquisitor.
So, they changed, although under supervision of the inquisition, to daemonhost and back and are accepted as Astartes.