Oy. Well, fortunately that part isn't that difficult, technically. However... as Visitor said, the organ is a completely different beast, giving the illusion of being the same since it has a keyboard. In fact, several of them, and the pedal board to boot. Well, not to boot, but to be played with very nearly the same skill and dexterity as the finger keyboards.
In addition to getting usable shoes (although some organists -- myself included -- usually play in soft slippers) get all the practice time that you possibly can on a local church instrument. The fingerings used are different, and the feet have to play in perfect unison with the hands, and the touch is completely different.
There is another gotcha: registration. That is to say, the selection of which ranks of pipes sound when you play on which keyboard or on the pedal board. With two months to learn, and probably no opportunity to play on the Carnegie instrument until late in the rehearsal process, I strongly recommend that you and your conductor, who presumably knows the sound he or she wants, get together with the regular organist and decide on what registrations to use where, ad learn how to set them up and change them as needed during the piece (the registration does change in the Mahler, by the way)(something else for your hands to do...)
Get a good page turner and assistant and practice with him or her. You'll need it.
Also be sure to get time to play on the Carnegie instrument, and try it. It sounds weird, but I have known people in your situation who simply froze solid the first time they came close to drawing the full organ. Trust me: a major organ like that one, in full cry, will let you know in no uncertain terms why it is called the King of Instruments. Remember -- you will have the power there to overcome the entire orchestra!