That may have just been an K.A. error (she's admitted to making a few significant ones over the course of the series, which is no great fault considering how many books there are).

As you said, it may have just been sentimental - he was promoted VERY rapidly, and so he may well have still thought of himself as 9466.

On the other hand, the number could reflect /social/ ranking rather than military rank, which might explain why he might have kept the designation even when he became Visser One. He had a high military rank, but pretty much everyone in the universe - including other Yeerks - hated his guts. Perhaps an unchanged designation would reflect this animosity, sort of like the President not signing off on a bill (if that makes any sense).

Personally, I like the idea that the name would have changed if K.A. had managed to catch that detail. That would mean every Yeerk would know exactly what their place was in the hierarchy of their entire species - and so out there, somewhere, there is the lowest ranked Yeerk ever. And they are never having a good day. That sort of small detail emphasizes how much drive and ambition the Yeerks have as a species, which meshes neatly with their theme and dilemma of a need for control and conquest.