According to John King (in his article "A New History of the Origins and Development of the 'Ukulele, 1838-1915" from 2003), the earliest ukulele teaching book (Ernest Ka'ai, 1910/1915) used C6. In the years following, various authors of method books altered between C6 and D6, but it seems that C6 (gCEA) is the traditional, perhaps the original tuning. So now that we're back to C6, one could say that D6 was a temporary exception only, possibly brought on by the desire for unamplified volume in entertainment places, though Dirk of Southcoast Ukuleles makes compelling arguments that D6 is better suited for small soprano bodies.
As for concerts, those were originally called tenors, and I believe there were various tunings for those. Cliff Edwards, who allegedly preferred that size, favored the Bb tuning (fBbDG), and it is used in some of his books. Noteworthy is probably that people used gut strings back then, which aren't quite like fluorocarbon strings, so some things that worked back then, may not work so well now, and vice versa. Bb on a concert is probably pretty floppy unless you get custom strings for it (it's kind of floppy on a tenor scale already, with Worth Browns anyway).
All of this may have been covered by the links above too. Didn't check them, so I apologize if I repeated the same information.