Book names appear to change on the large plates when there is a change in the dynasty. That is, when a new ruler is chosen who is not directly related to the previous ruler (as a brother or son would be). The new book seems to begin with the seating of the first ruler of the new dynasty. That doesn’t happen in the book of Helaman, and we don’t have Helaman seated until the second chapter.
The first point to note is that Helaman is the son of Helaman, not of Pahoran. There were several sons of Pahoran (Helaman 1:4), but only three contended for the judgment seat. Helaman wasn’t one of them. Therefore, when Helaman is seated, he creates a different dynasty. The fact that the records stayed with Alma may be the reason that the book of Alma was continued, even though Alma had abdicated his position as Chief Judge.
The second point is that the events of the first part of the original chapter of the book of Helaman (our Helaman Chapter 1) were perhaps part of the record in the book of Alma on the large plates. As discussed in the commentary on Alma 63, Mormon appears to have ended the book of Alma on the more significant forty-year date, rather than waiting for the beginning of the new dynasty in the less significant forty-second year.