This is the first, and will be the only, time that we have a change in official recordkeeper without also having a change in the name of the book. The evidence why books are named for specific individuals suggests that it is due to the creation of a new dynastic record. There may be multiple kings’ histories included in a single book, but only when those kings are part of the dynasty of the king for whom the book was named. In this book, the book of Alma, Alma’s eldest son Helaman becomes the new recordkeeper (he is not a king). We will eventually see a name change when moving from the book of Alma to the book of Helaman, but that Helaman will be the grandson of Alma.
This header indicates the shift in the original recordkeeper, rather than a change in dynasty. The next verses will explain why Alma is no longer the recordkeeper. Even though the header appears at this point, some of the events of the next verses precede the change of recordkeeper, which required this header. This header may have been on the large plates, unlike other chapter headers which are more clearly Mormon’s additions.