I doubt very much that the DC discounts will be automatically applied once the DC rates come out. I would call and have that discount applied. Yes, they will use the same ressie and just change your rate.
Pro - January is great for getting around the crowds. As a matter-of-fact, when we went Jan. 2001, it was so empty we thought we were locked in Epcot after the closing. We actually missed seeing people. Then again, if I was going with a group of 18, it would be perfect.
Pro - Jan. is value season and there is usually great discounts. AP is usually the best discount out there. It's usually worth it to pay the extra money for an AP, even if you're only planning on being in the parks for a few days, because you'll save so much on the room. Often 40% or more. Last year I paid $40 extra for my AP over the 7-day park hopper and saved $150/night on a GF LV room. With an AP, I believe you can reserve up to 3 rooms, so you might need 2 people in your party to purchase an AP for the 5 rooms, but the savings on 5 rooms could be tremendous.
Con - It can be extremely cold. DH wouldn't let me bring my winter coat, b/c he didn't believe Disney could get that cold. Boy was he wrong. We spent the whole week looking for gloves and earmuffs - The mornings were in the 30's and the afternoons sometimes reached the mid-50's.