(or "Self-Plagarism Is The Best Kind Of Plagarism")
For those of you unfamiliar with Kull, King of Atlantis, he's a character created by Robert E. Howard during the 1930s "pulp" era. He's a savage tribesman who wound up as a slave, then graduated to a gladiator, then got free and became a thief and mercenary. Finally, he worked his way into the palace guard and from there wound up becoming king due to the plots and intrigues of the decadent court, although his disdain for scheming and ultimately noble, if rough-edged ways resulted in a new era of prosperity. He fought against evil sorcerers and would-be conquerors with equal ferocity, and his dark hair and bronze, well-muscled arms marked him out against the pale, skinny Atlanteans. Basically, he's about the most obvious Conan rip-off you could possibly come up with...except, of course, that the same people created both, and Kull came first.
At that point, the question becomes, "Why did Howard create Conan?" After all, he already had a barbarian hero with a storytelling engine that allowed for a wide scope of fantasy stories, from sneaky capers involving fleet-footed thieves to sword-and-sorcery epics to tales of palace intrigue and treachery. They clearly couldn't have been that different; 'The Phoenix and the Sword', Conan's first story, is word-for-word in some places the same tale as the unpublished final Kull story, 'By This Axe, I Rule'. (Have I mentioned that subtlety was never Howard's strong point?) Thulsa Doom, who was used and re-used by generations of post-Howard writers as a Conan villain, is in fact out of the Kull stories. There really is only the slimmest of difference between the two men, and only slightly more in their storytelling engines. Atlantis is a vanished age to the Hyboreans, but does that really matter to a modern reader?
Ultimately, it seems like the decision came about as a result of commercial factors. The difference between Kull and Conan is a difference of emphasis more than anything else, with Conan's stories involving slightly more swordplay and battle and slightly less melancholic contemplation and dark magic (Kull's no philosopher, but he is more introspective than Conan...) But Kull never really caught the fancy of the editors of 'Weird Tales'. Howard seems to have decided that while he was retooling the series, he also needed to rebrand it to keep editors from rejecting the stories out of hand. 'Kull' had become damaged goods, and so Howard went ahead with a new name as well as a cosmetically-different setting when he changed the tone of his series.
In the end, it's hard to argue with the success of the move. It might well be that a a retooled Kull could have caught the public imagination, and certainly post-Howard storytellers have shown that a disjointed or convoluted timeline is no obstacle to the success of a storytelling engine ("missing adventures" and "definitive chronologies" being something of a cottage industry for Conan.) But clearly, the new character of Conan caught hold of the public imagination in a way that has kept the character going for decades and will probably keep him going for centuries. And as a side benefit, while Kull has never developed Conan's popularity, he has his own devoted following that spins off the occasional new series or movie for that character. They seem happy to follow the original Conan imitator...an imitation so old, in fact, that it predates the character it's imitating.