I think Xykon especially in Start of Darkness is a great example of how becoming a normal lich influences your alignment. That's the baseline assumption, I think, like Ghosts in Supernatural where the longer you remain as this inhuman entity, the less and less you care about your humanity; the more of a monster you become.

It is a common trope, and it fits here.

That said, it shouldn't be impossible to fight it or change. Barring of course Good Liches from Libris Mortis that RAW explicitly DO exist, an evil lich redeeming herself and doing good deeds and fighting her nature and all that would be a cool story, have potentially awesome RP, and would be something I'd love to be a part of.

At the end of the day, it should be a fight, though. She wouldn't just wake up and say "I'm Lawful Good now, lol, time for some Exalted Feats" but rather take a long and hard journey that never ends as the taint and influence never leaves.