I'm by no means a veteran writer, but I've done NaNoWriMo enough times that I have a good grip on its ups and downs. Stories I've written during NaNo generally break some, most, or all of the rules I normally set down for myself as a writer. It's a necessary part of NaNo - on such a tight schedule, I don't have time to be quite so picky about things like writing complete scenes, characters wandering off to do their own things, or their names changing.
In 2007, one of my primary character's names changed about a third of the way into the story, and I failed to notice the change until several thousand words later. Herbert, the somewhat bumbling pseudo-antagonist to the main character, inexplicably became Harvey, who was slightly less bumbling but more focused, more intent. Herbert did something that hurt someone else by accident, and was nervous about how he was going to escape punishment. Harvey, by contrast, knew he was doing wrong and did it anyway, because he felt he needed to. That his actions may have harmed someone was not nearly so much of a concern to him as it had been when he'd been named Herbert.
The accidental name change lent a bit of momentum to the story. The protagonist, not well defined, had mostly been bouncing in an incidental way from plot point to plot point. With Herbert's transformation into Harvey, she had more of a foil to work against. While Harvey wasn't exactly an antagonist, he led her into more adventures, I think, than the wimpier Herbert would have. And there's a nasty edge to Harvey that makes me glad to see him get (sort of) what's coming to him.
It's important to go with the flow during NaNoWriMo. If I'd done an accidental name change of a principal character in another story, I'd be much more distraught. But in NaNo, not only does it not matter during November, it's a NaNo novel, and the rules of 'you can junk it if you want' apply. I think Herbert should stay Harvey, and when I decide to work on this story again, I'll make the necessary changes in the manuscript.