User blog comment:Godisme/THE STAIRWAY TO HEAVEN/@comment-5551986-20121121005551/@comment-5341204-20121122070408

According to the page on Kiganjo, the Kenpachi that Zaraki ...succeeded..., he also got his captaincy by challenging his predecessor to a duel and killing him. Given the nature of the title, I think that it might only be attainable by killing the previous one, and if one was inducted into the Royal Guard, he'd likely keep the title anyway and the Gotei 13 wouldn't have that title anymore.

Of course, Kubo often deviates from what I would consider logical extrapolations of things we already know, so I could be wrong.