Iverson or Offerdahl

Could you help me find out about Lars J. Offerdahl born 27 MAR 1843 in Sogn, Oppland, Norway. I think his name before Lars was Lewis Iverson as I find that on records on the land here in South Dakota. His wife was Rigndlia Hovland Birth 24 Dec 1849 in Norway . I was told he changed his name to Offerdahl but do not know why and I would like to find his father and mother and siblings and when he came to the United States and why. Thanks