Except that she's not appearing as Red Sonja. Variety says she's playing "an evil half-human / half witch", presumably an ally of Stephan Lang's sadistic warlord, Khalar Singh. The plot revolves around Singh trying to track down a prophecy laden Queen whose sacrifice could summon him a demonic army. Having a witch around would be handy for that kind of work.
Will this be some kind of weird origin retcon, and McGowan's half-human / half-witch will end up becoming Red Sonja? I hope not. It would be terrible. Will McGowan be disguised under enough hideous makeup that it will simply be a fun bit of trivia that she appeared in both films? (I doubt that. Women aren't allowed to look unattractive in the Hyborian Age.) Will she be some kind of evil relation of Red Sonja? I'm certain I'm overthinking it, but I feel like it's no accident, and meant to be some kind of tie-in or Easter Egg. Nevertheless, I'm going to choose to believe it's another nail in the coffin for Rodriguez's Red Sonja, and that she's just appearing in the Conan universe as a good laugh and an admission of defeat.