Convert from epub3 to epub2 (yes, backwards)

I've been out of the loop for a while and I have gotten myself in a fix with this epub3 thing.

I'm creating an epub via export from InDesign. (I think that's where I went wrong by exporting as epub3) The goal is to upload it to KindleDirect. I've actually done this lots of times in the past.

Here's where I noticed I am using epub3. I spent several hour making things look right in Sigil (yay Sigil!) but then I go to delete the toc.xhtml file(whic was in the original epub exported from InDesign) and it tells me "The Nav document can not be removed". I had created an html toc using Tools>Table of Contents>Create HTML Table of Contents.

So as I understand it, epub2 is what I want for KindleDirect. And I really don't want to use that toc.xhtml file.

I unzipped my file and changed this line in the content.opf
and changed the 3.0 to 2.0. I re-zipped it and opened it in Sigil. Then I was allowed to delete the toc.xhtml file no problem.

I still have a toc.ncx file.

So here are my questions

1) is there someway to convert my document back to epub2 within Sigil? Is seems like there should be a thread about this but I couldn't find it.

2) If there isn't, Is my unzip-edit-rezip method OK?

For several reasons, I really don't want to start over exporting from InDesign.

Standard apologies if this has been covered in another thread. If you could point to one I'd appreciate it.

Thanks! And Thanks to everyone who has kept up with Sigil development.

BKh