@kovid, A couple of comments about this new feature...
- Might it also be an idea to rename all text files to have the .xhtml extension as part of the update? An EPUB2 which epubcheck says is error-free, will give errors (warning only) for any .html files in the EPUB3 after the update. (I know you don't rate epubcheck so please don't shoot the messenger
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- If users are updating to EPUB3 so they can use some EPUB3 features (e.g. epub:type=...) it might be useful to auto-add the epub namespace to the <html> tag of every text file as part of the update.