They ceased to exist as distinct entities.
(In fact, the terms "Nephite" and "Lamanite" as used throughout the Book of Mormon are different from how they are used at the end. At first they were two different ethnic groups, but they basicall merged, then later this single group split on religious lines, and each faction used the name of a former group. Thus "Lamanite" as we use it to refer to the people of 400 CE means "descendant of Nephites+Lamanites".)
(In fact, the terms "Nephite" and "Lamanite" as used throughout the Book of Mormon are different from how they are used at the end. At first they were two different ethnic groups, but they basicall merged, then later this single group split on religious lines, and each faction used the name of a former group. Thus "Lamanite" as we use it to refer to the people of 400 CE means "descendant of Nephites+Lamanites".)