I've used this series of search & replaces to convert from single curly quotes to double curly quotes. Saved as a saved search in Sigil an then load all/ replace all. Use with caution until you are sure it's doing what you expect. It's not perfect but gets most of them - expects the text to be decent punctuation in the first place.
Quote:
Name=Quotes
Left Quote
Find=<p class="calibre(\d+)\">‘
Replace=<p class="calibre\1">“
Left quote2
Find=(\p{P}) ‘
Replace=\1 “
Right Quote
Find=(\p{P})’</p>
Replace=\1”</p>
Right Quote2
Find=(\p{P})’
Replace=\1”
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The Right Quote2 have a space after the end of find and replace.