If all of the chapters contain exactly the same
Code:
<p class="calibre10"><span class="calibre8 bold calibre11 calibre8">
and then the chapter number followed by
then search for (regex, minimal)
Code:
<p class="calibre10"><span class="calibre8 bold calibre11 calibre8">(\d+)</span></p>
replace
Code:
<h2>Chapter \1</h2>