If you're worried about losing the good stuff on a second go, I'd say just go in for a complete rewrite like onesecondglance suggests, and once you have that, go back through the original and take out anything you think must be in the new draft, and if it isn't more or less there already, see if you can work it in. I find in the standard drafting process that I go through adding large chunks anyway, sometimes whole chapters, that it shouldn't be hard. Maybe in some cases it will require considerable reworking of a section to fit those details in, but it can be done.
Of course, maybe when you get to the point of transferring details from the original to the rewrite, you may find that some of those details aren't as important as you would have thought. As Stephen King wrote in On Writing “Kill your darlings, kill your darlings, even when it breaks your egocentric little scribbler’s heart, kill your darlings” because sometimes what we writers like and cling onto don't actually belong.
And, yes, I'll second that complete rewrites can be helpful. I haven't done it on a novel length work yet, but I've done it a number of times on short stories and it's allowed me to transfer a good core idea from a work that isn't written a quarter as well as I write now (however good that actually is).
You may, however, want to experiment with what you can do with just extensive drafting, especially if this is your first novel and you haven't done much drafting (I don't know what you've written before). I don't know how far you've gone before so this may be redundant, but for me sometimes the drafting process involves completely tearing apart a scene, starting almost a new but dropping in a still useful paragraph or sentence or phrase from the previous draft. And of course as long as you're comfortable adding scenes and chapters, and taking others out, if you draft heavily enough you can find that the new version is almost, if not quite, like a rewrite.