It depends on the motive - during the Civil War if you were in the wrong town or state, you may have been in harms way, so pick a safer location; if not in person at least in spirit. The areas are similar and if you were a Northern sympathizer in a Southern town, whoa. Sometimes it was the change of boundaries by the government - some Virginians ended up West Virginians, Texans ended up Mexican, and in some cases, the location was a guess by location - along a river or lake, small mountain, or big forest - they know it was on the South Carolina side but later it was anexed in Georgia; that type of thing. There was also the idea that the location could be 2nd hand; somebody guessed the location based on "their" knowledge. Take Prussia, most of the beginnings were listed as Germany, during the war evolved into Prussia, finally ending up as Poland. The land did not move but it became 3 different countries in at least 3 different census listings. So, actually it is not common to change, but it does happen more often than not.