In order to preserve the blood flow to the nipple and areola, surgeons have to the blood vessels intact along a "pedicle." There are a number of different pedicles that can be used to do that, but some of the pedicle have a more robust blood supply than others. I would agree that preserving the blood flow to the nipple-areolar complex, in your case, would be difficult using a superiorly-based pedicle. Using a superiorly-based pedicle and trying to go from a G to a D would be challenging, so I understand your surgeon's caution and recommendation for a free nipple graft in that case. If a different pedicle is used, such as an inferiorly-based pedicle, potentially more breast tissue can be removed, making it possible to go from a G down to a D. The tradeoff though is that the inferiorly-based blood supply is not as robust, so the surgeon might putting the blood supply to the nipple and areola at risk if that approach were used. If the blood flow is disrupted too significantly, the surgeon might have to covert to a free nipple graft. So your surgeon certainly isn't wrong for recommending the free nipple graft, but if you seek another surgeon's opinion, I would make sure that they are comfortable with breast reductions and have used various pedicles before in the past.