I am a Calhoon by marriage (The family is from Kentucky, but originated in W. Virginia, I believe) and the story that my husband family subscribes to is that the spelling change came as a result of the slavery issue. They claim that the Calhoon's did not have slaves and that they changed the name to disassociate from the slave owner families.
I'm not entirely convinced that's true, as my husband's great great uncle had "negro's" listed in his will, but since he left property to them, I have a hunch they may not have been slaves, but descendents. My husbands' family are Irish and Scot. (tracing to Robert the Bruce, as a matter of fact) and with NO hint of German at all.
I have a spelling change in my maternal grandmother's name as well, and that dates to the Revolutionary war. They ended up on the wrong side and moved to Canada and changed the last part of the last name from "steen" to "sted".
Another reason, might have been a literacy issue and not intentional at all.