This is probably so simple that it's been solved a dozen times, but I'm
stuck. How do I remove quote marks from content that are interfering
with correct parsing of XML?

I have a lousy translation to XML(XHTML) that has decided that all
quotation marks need to be translated as ".

Which of course leaves me with text that looks a bit like this:

<p>I would <span class="annoy_reader">never</span> reveal the name of
the <span class="so_called">"quality"</span> product that put me in
this position, but it lacks a certain flare.</p>

What I want to do is make sure I replace the quotes around quality
with... well, anything except what they currently are, really (but in
this case, &#8220; and &#8221; respectively). And of course, I want to
leave the other quote marks alone, since they're correct.

This has to be such a common problem that solutions exist by the
handful, yes?