For me, the dividing line is the departure of Sonny Greer. Johnny Hodges left, but his replacements were variations on a theme , and anyway, he came back. Greer's replacements were always more "modern" in style and once Sam Woodyard settled in, that was it for the duration .
I might even think that "late" Ellington begins with the departure of Woodyard and the appearance of Rufus Jones.
The "character" of so many big bands is set my the drummer, and Ellington's was no exception. Of course, that's not the only thing, but it's a fundamental one.
In terms of records, there was a period of transition between the last Columbia with Greer and the first with Woodyard . The Bellson period and further are perhaps the definition of "transitional".
It's definitely one of the longest uninterrupted journeys in musical history.