"What do you writers think?" Dave asks. "Can a [character's] voice just go away?"
I think it depends what kind of voice the character has in the first place. A distinctive, intimate, first-person voice can be tough to rediscover after writing others. By comparison, third-person characters rely less on voice.
A writer's method of drafting stories is another factor. A story can only be planned out so far. Its spontaneity comes from the voice(s) telling the story. Voices eventually deviate from plans, and if the writer doesn't follow, the story is often worse for it.
Like any style choice, there are pros and cons. You can probably stay in a character's voice if you write about him exclusively, but if you do, your overall writing may not develop much beyond that character. On the other hand, if you don't write a character for a long while--years, say--you may lose his voice.
I find it helps to make characters as well-rounded as possible, give them room to move within the lines that initially define them. The old adage is characters are consistent while people are not, but completely consistent characters--always do this, always say that--eventually get stale.
"What's he going to do in Book 15?"
Pretty much what he's done since Book 10.
I face the same issue whenever I go back to C.J. Stone. To work around it, I've made C.J. an unreliable narrator. Because the stories he tells most likely aren't the whole truth, I can change or discard details that don't serve future stories.