To be nitpicky, Ed Hamilton took over as full-time Legion scripter all the way back in Adventure #306, and he held that title for the next forty issues. The handful of remaining Legion stories by Jerry Siegel over the next few years were the fill-ins. It was Hamilton's retirement from comics writing (combined with Siegel's departure to pursue another legal action against DC) that made editor Mort Weisinger receptive to the idea of bringing aboard thirteen-year-old Jim Shooter as a new writer.
(And perhaps this is the appropriate place to note that I wrote an essay on the odd relationship of Siegel and Hamilton with Weisinger for the LSH-themed anthology Teenagers from the Future, so I've spent more time contemplating this than is strictly healthy.)