Yeah, I don't get it. Lissandra already had pretty solid lore too. She barely needed the update. And its a shame because I really like the short story that came out with it: It gives a lot of creepy Void vibes and also fleshes out the Frostguard themselves and how they function and fit in the world.
These changes are just confusing. How did they think she needed to be changed? Why would she try to erase/manipulate history now? Why does it feel like Avarosa's contributions suffer because of this?
All they had to do was hit the high points: Watchers make them Iceborn, Lissandra worships them, sisters disagree, Civil War ends with Lissandra losing and Watchers in Howling Abyss, Lissandra plots their return with her "plans within plans" style that made her so iconic, now she leads the Frostguard. I wouldn't even care about the whole "Avarosa is deaf, Serylda is mute" bit or any other odd details if they had just stuck to these few points.
These few points that, mind you, already fleshed out all of Freljord and most of its champions.
As you said, Lissandra's appeal comes from being a true Villain, firmly on the side of an entity that will bury the world in ice.