As I've rambled on about for some time now, I'm currently in the process of doing a major rewrite of my novel. A big chunk of this involves totally new chapters, interleaved with the existing material. The second half of the book is going to get reworked pretty well from scratch, but the first half has a lot of decent stuff that just needed some tuning.
It's interesting to see, though, how much more positive feedback I'm getting on the new chapters versus the old. This isn't the first time I've had this happen either. One of my earlier projects was actually a fantasy trilogy, which I did draft from start to finish. My problem once I got to the end, though, was the disparity in the writing quality between the books. I'd spent so much time working on the whole thing, I'd grown substantially as a writer. In fact, the gap was significant enough that I've bottom-drawered the whole thing. If it ever sees the light of day, it'll be in an entirely different form.
That's kind of the dilemma, though, in writing long fiction. When you're still getting to grips with the craft, still learning how to make a story really clip along, every six months or a year brings with it a leap. The new stuff might not just feel better than the old--if you're paying attention, it probably is!
At this stage, I think the first half will be workable without dropping it, but that doesn't mean I don't feel the tug, hear that little voice in my head that says, "If you rewrote it all, it'd be better!"