I imagine she spent a while trying to figure out what could possibly have beaten her unbeatable curse, and eventually got to "Well, it's a body-affecting curse, so maybe changing the body does it"? She's quite sharp, is Bellatrix. So then she went and did research....

And she's known for some time. That was ages ago, after all. I don't know if the Order knows she knows, though.

Draco commenting on his ability to keep secrets such as that one is in his ISS intro.