As I'm reading your book, I'm curious about what other aspects changed. Has the tone remained the same and you've just tweaked plot points and characters? How different is the book from what you'd originally envisioned?
One thing I've always said about writing is that as an author, you shouldn't get too attached or fall too in love with any passage or particular part of your prose, and I think that this attitude served me well when it came to TDLF revisions. It's very easy to think that what you've written is genius, but writing a decent book isn't and shouldn't be easy, and thus the harder decision often isn't what to say, but what to leave out or what to cut.
So, with that out of the way, to your question. The tone has essentially remained the same. I "got" Natalie's voice from the get-go, and her voice is such a critical part of her story and her evolution that I never dreamed of - and more importantly, no one suggested - that I modify it.
That said, as I've mentioned before, I certainly made other drastic changes. After reading the first draft, my then-agent suggested that I might have started at the wrong point of the story - I'd originally started on the day that Natalie was diagnosed - and guess what? Turned out she was right.
So out when the first 90 pages. Easy come, easy go. Even though yes, I'll admit that it was a tad painful hitting "delete" on all of those pages. Then, we decided that Natalie needed to be more likable, more relatable. So I went back and threaded in the entire Price is Right plot line. Yes, it's true, the aspect of the book that seems to be among peoples' favorites wasn't even in there to begin with! I also went back and added touches and hints about Natalie's relationship with her mom: why Natalie was who she was and how she got that way.
And this doesn't even cover the stuff that shifted in my mind while I was writing the first draft: the shifting love interests, the role of Susanna Taylor (who I didn't realize would play such a pivotal role until I was writing), the friendship issues (won't elaborate too much on those, in case of spoilage). These all came up organically as the story took me where it did.
So, the bottom line of this post is really to say, go with the flow. I think often times, authors lose any chance of objectivity because they so passionately believe that what they wrote is solid gold. But there's a reason that we have critique partners and editors and agents. And that reason is because they make our writing better. So let them.
How do you guys feel about the editing process? Is it painful and ego-blowing or a means to a better product?