What if they just misprinted the name in the past with berenstein, didn't care to change it ,and later on re-printed with the berenstain name?
That wouldn't be worthy of a thread. But no, that's not what happened. The change is retroactive. I.e. people who had those books, say, stashed away in the attic from way back in their childhood, climbed up to that attic, or alternatively descended into the basement, and discovered the new spelling on the old books, different from the old spelling they remember on those same books. Thousands of people.