when re-converting, save previously edited TOC?

I'm working on a group of ebooks, and learning Calibre as I go.

I'm converting from html to other forms, especially mobi or AZW3, and I have to manually tweak the TOC during conversion. This is a somewhat time consuming process, since I'm still learning and sometimes (often???) have to re-convert a book multiple times to get it right.

thankfully, the metadata stays, but the manually tweaked TOC has to be re-tweaked every time.

Is there a way to retain the TOC that I had to create by hand, when re-converting?

thanks!