It seems not OK 冬天更换屋顶. I remembered my contractor told me that the temperature should be in the range 10-25 degree, not too hot, and not too cold for a quality work.
Actually, you can ask the seller either to deduct the equivalent price, or to give you the same amount cash for you to fix yourself (around $3000-5000 for a roof work). Unless he could DIY.
The warrant usually lasts for 25-40 years, but the roof is really for around 15 years only.