Originally Posted by Manichean
Why do you assume that it uses the MOBI format in the case you provided screenshots for? I see a book entry containing AZW and MOBI, and I see a warning that the destination format (MOBI) already exists and will be replaced by the product of the conversion. I see that your most preferred input format is AZW, thus, I assume that Calibre will use that format as source. Is there anything indicating that Calibre wants to use MOBI as a source format?
Because, if I convert an individual
book and it has any format as well as already has MOBI, if I convert from something else to MOBI, that "Convert Existing" popup does not appear. If I tell same individual book to convert from MOBI to MOBI, it does.
Therefore, I assume that the "Convert Existing" popup exists to warn me that I am using an Input format to overwrite itself to the same Output format (which makes sense, because if that were the only format of the book available, then the changes become permanent.)