Thanks all, for jolting my jaded brain and alerting me to options I hadn't considered

Your personal plug is most welcome, Dawn, I wish I'd known about your group before. I have signed up. It makes sense, because most of us must be stuck into a novel already, and are loath to tear ourselves away from it and start a completely new project.

I had written 20 chapters (about 60k words) before I started rewriting mine (it should come out shorter this time, 100K in total, I think).

I didn't know NaNo allowed you to 'manually' deduct the word count of chapters you had written beforehand. Doing this doesn't make you a 'rebel' because you have negated any unfair advantage you would have had over writers writing from scratch.

'Copy-typing' chapters already written is another option. I doubt if anyone could manage this without spotting an error and thinking of better forms of words.