Spell Store functions as it did before when self-cast. When cast upon a TARGET the spell becomes a spell-stealing utility for whatever magic was being prepped by the TARGET. The spell works with a successful ward over +1 point and will immediately prepare the spell which was taken as if the Wizard was knowledgeable from that spell circle.
This will be a very convenient spell for Wizards to have in their arsenal because it can counteract a lot of very dangerous spells and conversely turn them into a tactical advantage for themselves. It would also make a spell slot highly utilitarian rather than almost never used!