Late answer: FVector2D has a constructor that accepts an FVector reference and creates a new FVector2D from the passed in vector’s X & Y components.
FVector Vec3D(1.0f, 2.0f, 3.0f);
FVector2D Vec2D(Vec3D); // Results in FVector2D(1.0f, 2.0f)
A similar conversion (via constructor) exists the other way around as well.