Not magazine specific, but why not. This is from the Frazetta Museum facebook page posted by William Frazetta.
"Frazetta's original cover painting for 'Conan the Buccaneer' was loosely inspired by one of his own illustrations for the Canaveral Press edition of 'Tarzan and the Castaways'. Certainly a far more complex composition than many of his other book covers of the same time period, Frank's painting electrified audiences and revitalized sales of the flagging 'Conan' series. Fans rejoiced, the publisher was ecstatic, but Frazetta was not satisfied: something just didn't look right. When the original was returned to him Frank repainted the barbarian's face and helmet, adding armor to the figure and detail to the necklace. Still sensing that the work was unbalanced he ultimately elected to remove the barbarian entirely and start over. The painting in its present form is entitled 'The Destroyer' and is easily one of Frazetta's most popular works. Neither version shown on these pages stil exists. [Fenner] still like[d] the first rendering a great deal, almost as much as the revised canvas, and said as much to Frank during a visit to his home. Without turning his head, he shot [Fenner] a sidelong glance and growled simply, "It was ."
Excerpt from 'Testament' (Fenner, 2001).
My grandfather would often change pieces of artwork when they returned from the publisher, and I always get questions (often asked in frustration) as to why he would do it. There could be a few different reasons: first, he knew he had to do them for a publication, so certain things were off-limits (ie nudity such as in Cat Girl); second, in the case of 'The Destroyer', he liked to make sure most of his paintings gave each side a fighting chance. Originally, the barbarian was choking someone in the middle of a battlefield, unarmed. Sure, Frazetta painted fantasy, a genre in which anything is possible, but he still liked to err on the side of reality-- no one was immortal, and that included the barbarian. The third major reason for his changes, and the most important one, is that he just simply didn't like the first version. Whether it was due to licensing constraints or "bad" ideas, the first one didn't speak to him, so to make himself happy he'd redo it.