title_body: Which is the quickest way to change apostrophes to quotation marks? I am proofreading a text, and I would like to change all occurrences of ' to the relevant character, ‘ or ’; ditto per " and double quotes. 
  The task cannot be automated, because there are apostrophes in the middle of the text, like in "I'm", and of course " is heavily used in the XHTML markup. At the moment I am using Sigil, but replacing every single character takes a bit of time because of the tidying involved. Do you know of any better way?
upvoted_answer: You could use the PunctuationSmarten Sigil plugin.