The Clone Wars Returns

Animatics for 'The Bad Batch' and 'Ahsoka in the Underworld' arcs were produced at some point in 2012 most likely (possibly 2013 but it'd be a slim window of time since the show ceased production in March of that year). Compared to the stories as they were at that time, there are a few differences. The Bad Batch had a few minor tweaks in dialogue ordering in most scenes, changing emphasis slightly. A few scenes were deleted for pacing reasons, though that seems to be a pretty normal part of the process. Three scenes were explicitly added in the 2020 cut, those being the initial one with Rex and Cody commiserating about their lost colleagues, Anakin's meeting with Padme (replacing the cut 'nose art' scene), and an alternate ending with a more sombre tone.

As for the Ahsoka arc, this one was modified by replacing the character of Nyx Okami with the Martez sisters. Ahsoka's outfit was changed to overalls. The basic plot of these episodes remained almost exactly the same though, as evidenced by the original concept art. Bo-Katan and the Mandalorians' appearance was a new embellishment, as was the Maul scene, so presumably they would have run into each other in a different arc (Sith Shrine most likely).

The Siege of Mandalore was only at a script drafting stage back in 2013, with concept storyboards. The Ahsoka novel's prologue also seems to be based on these same early drafts, though that's not completely for sure. These indicate some changes into the final draft. Ahsoka's sabers remain green (as does her outfit, matching her Season 3 Jedi outfit, probably just as a temp before the design was finalised). Maul is seen conferring with Almec during the invasion, whereas in the 2020 cut he's left offscreen until the first episode's climax. If the Ahsoka novel details are accurate, then Maul was captured in a trap involving ray shields rather than the duel high up in the roof of Sundari. The arc also originally ended on Mandalore itself instead of the Venator, with Ahsoka and Rex escaping on wolf-back (that was probably changed due the Loth Wolves' use in Rebels).

Everything from the original production seems to have been overseen by Lucas, so for the first two arcs the changes aren't too major. Siege seems to have had some restructuring, likely due to going through further drafts in the interim, as it was in an early phase of development. The extent of how things changed is also difficult to ascertain, as not much was revealed about Siege and there's less concept art.

Incidentally, Dark Disciple and Son of Dathomir also seemed to have very few changes from their planned 2013 iterations, with Son of Dathomir essentially being a storyboard guide to the story.

Thanks fellas. I found this too:

I've said it before, and I very much agree with this. I'm not a fan of Matt Lanter. Lucas once said about casting Hayden in the role that he didn't want to cast a funny guy and try to make him act dark. He also nailed the casting with the 2D Clone Wars Anakin, Mat Lucas. He sounded like Hayden and had the right kind of youthful, slightly grating sound to his voice. With Lanter, he cast the funny guy, and I never fully bought his darker moments. He's too casual and out going most of the time. I think he lacks the gravitas that Anakin is supposed to embody.

I never liked TCW Anakin. Especially the voice. Seemed like Lucas was bending to the fans a bit, considering some of the criticism. "They didn't like the character." So he wanted to try to make him more "relatable".
The Anakin from the movies might have been TOO dark and moody for a kids action hero type show. Also, the Anakin voice from the 2003 Clone Wars sounds nothing like Hayden.