Old Lissandra entombed?
I just wanted to put my 2 cents out on the 
update, hopefully this helps anyone confused or that has mixed feelings about the update. I am not going to guarantee that you will like the update after reading this but hopefully it helps you appreciate the story even if you dislike it.
Basically old Lissandra still exists, the old Lissandra that supported the coming of the Watchers and wanted to rule a frozen world, canonically existed on Runeterra. From her bio:
With her sight taken, Lissandra chose instead to walk in dreams. As she navigated the fitful visions of those around her, she realized only she could see the darkness below for what it was: the lingering abyss promised not only an ending, but infinity. It was death, both dangerous and full of potential. Unknown to her sisters, Lissandra struck a deal on their behalf with the god-like entities she had communed with—the Watchers would grant them near-immortality in exchange for preparing Runeterra for the coming of the Void.
This was the Lissandra we knew from the old lore. The Rioters then sneaked in a weakness in Lissandra's story, her sisters, Avarosa and Serylda, the only 2 people she cared for. So when the Watchers sent the voidborn to Runeterrra, this is likely when the sisters went to 
and likely around the time Serylda tried to steal/ coerce power from Targon and Avarosa lost her hearing to the madness of the Void. So Lissandra began trying to convince her sisters.
However, Lissandra’s sisters grew displeased. Avarosa argued that the only thing worse than death was servitude. Even Serylda bristled against what would become of the world they had fought so hard for. Caught in the middle, Lissandra tried to soothe her sisters’ concerns while appealing to the Watchers for more time, but the unknowable nothingness cared not for such platitudes.
The Void erupted into the mortal world in the far north, and with it, Lissandra’s hidden allegiance to the Watchers became undeniable. In that moment, her only choices were to let all the world be consumed, or to give up what she cared for most—Lissandra sacrificed her sisters and the allies they had gathered, entombing the Watchers beneath a glacial barrier of magical ice that could never be melted.
So she couldn't convince her sisters, and the realisation sunk in (perhaps it were her sisters who convinced her) that surviving in a world ruled by the Void was a pointless dream. In old Lissandra fashion she ends ups killing everyone, Iceborn and Voidborn alike, to conceal her involvement with the Watchers from the rest of the Freljord, had anyone knew she was the cause for the Void war, they would have had a massive witch hunt throughout the Freljord. Now she is living the rest of her days trying to find a way to prevent the Watchers' return, as the Watchers taunt her for her betrayal from beyond the ice.
Admittedly, Riot released the wrong short story with the Lissandra update, imo. While Into the Abyss is an amazing worldbuilding story that explores the culture of the Frostguard. For Lissandra's update the story they should have released, should have been the 'Void War arc'where Lissandra first interacted with the Watchers and ultimately betrayed both the Watchers and the Iceborn. From The Iceborn's post WAAARGHbobo confirmed that it is a story they plan to tell in the future, understandably the likely reason it was not told now, is because it is too early for Watchers lore, as Vel'koz is the only voidborn with a full bio and short story.
Sorry for the long post, the main reason I wrote this is because, after reading her lore, my first thoughts were "New Lissandra is dumb! She switched sides so fast it makes no sense." this is coming from a non Lissandra main who thought "Cover the world in ice, sounds like a ridiculous plan." Watching TB Skyen's video helped me understand Lissandra's new bio in this fashion as well as the Void update that came with 
's bio. You don't have to like the new story (or any story tbh), but I think this change to Lissandra is more of a evolution of her character from the old lore opposed to a retcon.