I was wondering how it looked like. I play since 8th edition armybook and after reading about army of Slann on Wikipedia I was thinking how it was explained that Lustria changed. For me it looks clear that previous view of Lizardmen, or rather Slann was because of sailors tales, especially about this one Slann mentioned in armybook who likes lobotomy In fact old miniatures of Slann looks exactly like if someone was describing some big old frog who tried to kill him, or like if someone wanted to tell about big old frog met far away from home but the story wasn't thrilling enough to be told in tavern. Of course we had such things in history - europeans view of americans, ancient greeks and Hyperboreia. Have you thought about such fluff behind old Slanns? Did GW used this as explanation?
I remember Slann making a few appearances in the black library during the days of 4th edition, before anyone knew Lizardmen was going to be an army. I remember one awesome short story where a Slann is up to a lot of tricks, conning a lot of people in pursuit of power, it turns out old mate had cast a spell to give him the resemblance of a slann.
From th black library, I only remember small fragments, such as a slann wanting to eat a human visitor since he had never tasted human. Other than that, I do have the 5th edition armybook, as many other here probably have. The lore in it is largely the same through 6th and 7th, as I recall. This was also the time when the slanns were reinvented to the fat lazy frogs they are today.
5th edition Lizardman WFB also has a very different take on the Old Ones' intention for their created races - humans et al were actually intended to oppose Chaos and ultimately win throughout the universe! It is clear the Old One were destroyed in the Great Catastrophe no "mystery" Page 22 clearly states the Old Ones intended the races they created (elves, dwarves, humans etc.) to surpass the lizard races and to evolve and advance to the point they would vanquish Chaos entirely not just in the Old Word or on this planet but throughout the universe, but the Slann don't understand this aspect of the Great Plan. Whether this can happen without the Old Ones is not stated. Rob