I seem t recall Jamil saying that the Grand Cru was the result of extended aging (cant remember how long) and that the standard was a blend of the really sour aged beer and "fresh" beer that hadn't had a chance to sour so much. Jugglin with blending different levels of sourness to get consistency.
So I suppose the Grand Cru would just need to be aged completely out.
.... hang on, Jamil also said that he had designed his recipe to end up somewhere inbetween the standard and the GC, it had to do with how many dextrines he left in the wort for the weird bugs to chew on....
So I guess that you would need to use a different technique, if not a different recipe, to get the grand cru.
And so I have been no help at all.

Sorry
Thirsty