While I've said numerous times that the BMC as characters are my least favorite thing about the BMC arc in the manga and that that's one aspect R has over it, no they shouldn't be changed. Not if they're "trying" to stay as accurate to the manga's pacing and story as much as possible, to attempt any such change would just leave us with another Shittenou situation on our hands. Half-assed, short-baked, never went anywhere and completely and utterly pointless.
I love the BMC in the 90's anime, they're one of, if not my absolute favorite villainous organization. But the BMC in the 90's anime are totally different characters from the ones in the manga. Literally the only thing they have in common is sharing the same names, faces and powers. Everything else - personality, backstory, motivation, etc is completely different. So it would be disingenuous to try to give any of them a redemption arc when these are not the same characters who were never meant to be sympathetic in the first place. The BMC in the manga are scum through and through and deserve to die, period. Whereas in the 90's anime and some old musicals they were an oppressed nation of freedom fighters, in the Manga they are a hyper-religious literal death cult.
We see Manga Demande strangling a literal child to death, I doubt 90's anime Demande or Myu Demande would ever do such a thing (Especially Myu Demande since in the old musicals he takes two half-Nemesian half Earthling twin little girls into his clan)We see him outright state that "Love and Peace are an illusion." He genuinely believes peace is a bad thing and that war should always rage on, he literally tries to destroy the entire freaking universe and Space Time Continuum and you somehow think this guy deserves redemption? Also the rapey vibes he gives off are toned wayy the [BLEEP] up in the Manga/Crystal since it's clear that unlike the 90's anime where he was genuinely in love with NQS/Sailor Moon (despite how misguided he was in that love) In the Manga/Crystal he saw her solely as a conquest. He wasn't trying to own her because he loved her but to exert his dominance over her, he was planning to kill her after he had had his way with her.
I'm pretty sure the reason why Demande wasn't made sympathetic in the slightest in the manga was because he was supposed to be a commentary on toxic masculinity and rape culture, he is literally the face of it. And that makes it all the more powerful when Usagi is able to stare him down in the face and escape from his grasp using her own power and the help of her best female friends. Compare that to the 90's anime where Tux has to fly in to save her like she's some kind of damsel. It's clear to me that while in the 90's anime Demande was meant to be an actual character, in the Manga he's meant to be more of a representation, a good way of teaching young tween girls about female empowerment and the dangers of a Toxically Masculine attitude. Demande's extreme violence, belief that peace and tranquility is an illusion and for the weak and that love can be taken by force? All textbook examples of Toxic Masculinity at play.
Personally this is why Crystal's changing of his death is one of the changes that angers me the most. What exactly did it accomplish? I don't know about you but that change didn't make Demande come off anymore sympathetic one iota. He's still ultimately "protecting" her so he can kill her himself, he literally says it. So all we got in exchange was Death Phantom offing Demande himself rather than the powerful message that was supposed to be conveyed of Usagi killing her rapist alongside her consensual lover who she chose herself. It was yet another example of Crystal's stupid, useless and meaningless changes and they would've been better off just sticking to the manga.
And like others have said, even if you wanted to redeem other members of the clan like the sisters, you can't because they are literal cold-blooded murderers, in the 90's anime they never murder anybody. Hell, in the 90's the BMC's plots barely harm any civilians in general, if they're not chasing down Rabbit they're trying to infuse the Crystal Points with Dark Energy, they never attempt any bodily harm on ordinary civilians. In fact correct me if I'm wrong, but they might actually be the only villainous organization that does the least amount of harm to civilians in general in the anime? Far cry from their manga counterparts who are actually the only villains we see murder children and attempt rape.
But if you must redeem any Manga/Crystal BMC member might I suggest Rubeus? He actually ends up looking like the most decent one of the lot (ironic, considering his 90's counterpart was definitely the most dickish - in this case I'd say he's the only clan member who's Manga self is better and more fleshed out than his 90's version) He has a jovial, laid-back personality in the manga and treats the sisters quite decently, even seeming to have a bit of a fling with Calveras. He's also the only one of the Clan that's immediately onto Wiseman and tries to warn Demande before Wiseman axes him. I don't think any of the Manga/Crystal BMC deserve redemption or a sympathetic backstory but if I had to pick, Rubeus would be the one that makes the most sense.