i would like to ask, if i send my iphone for warranty will they change my s/n? As far i know, they will change my S/N to a serial that indicates my iphone is a refurbished phone. True?

One more question is, if i get back my phone from warranty, they will extend my warranty by adding additional 3 months warranty period? What if i just bought it today and tomolo bring back warranty, It came back 4 weeks later, my warranty period will it be 1 year and 2 months ? (deduct the 1 month because of the warranty process)