Your assumption is more than likely 100% correct. Those that CHANGED the name to Cohan did so because it was spelled that way by most people that heard it.
The census taker in 1870 heard the family name was KEOHANE and wrote it down that way. I stated it incorrectly earlier. 1870 was KEOHANE 1880 it's COHAN.
I have a map of Weymouth Landing in 1876 that list all the names of the people occupying each house drawn on each street. Most, all but one of five, are spelled COHAN. One is KEOHAN and they happen to live next door to my GGGrandfather J (O)Connor who used the O but many times it was taken as a middle initial.
I am revisiting the census to see if all these families could read and write. So far three sets of parents can not read nor write and some of the older children also can not. SO, it seems they were at the mercy of the enumerator or census taker as to how the name was spelled.
Hey we may be cousins or something, In the early 1950's I was a member of the Wesagussett (?sp) Yacht Club in North Weymouth and Don KENT the weather man, also the carpet store owner, was also a member. More later.