She just had Minerva transform, with no explanation, like she'd never lost the ability and had been doing it all along - totally ignoring any stories that indicated otherwise (including her own in BIRDS, amusingly enough), just as those stories themselves ignored that she had the ability.
It's still a bit odd because she seems to have dropped the "wracked with pain when in human shape" bit, which there's no precedent for. But that's easily enough handwaved by the Circe involvement.