I want Zoro to keep Sandai Kitetsu for the remainder of the series. I see no point in moving up the Kitetsu lines, as we're nearing the end of the series, according to Oda. And if the idea is that Zoro has to upgrade to the Nidai Kitetsu, then logically he should move up to the Shodai Kitetsu, but that would be even closer to the end.
Meanwhile, he's had the Sandai Kitetsu for over 800 chapters, and it's the one sword you can say is patently Zoro's. He didn't inherit it from a previous user like Wado Ichimonji, Shusui, or Enma. When you think about the Sandai Kitetsu, you won't instinctively think of another user like the 3 swords I mentioned. Plus there's something inspiring that the weakest of the Kitetsu will end up being the most renown under the hands of the future WSS.