Full Idea
While there is indeed a before-after difference in Mary, this is just a matter of coming to think in new ways, and acquiring a new concept. There is no new experiential property. She could think about the property perfectly well, using material concepts.
Gist of Idea
Mary acquires new concepts; she previously thought about the same property using material concepts
Source
David Papineau (Thinking about Consciousness [2002], 2.2)
Book Reference
Papineau,David: 'Thinking about Consciousness' [OUP 2004], p.51
A Reaction
I think it is better to talk of Mary encountering a new mode of experiencing something, just as experience becomes blurred when glasses are removed. No one acquires new 'knowledge' of blurred objects when they remove their glasses.