Originally Posted by cybmole
i don't follow your examples - sorry.
can you try again with a "for dummies version" please.
and yet—and yet
a bur—a hobbit
Both have an unspaced em-dash, but the first one should (could) be replaced with "space en-dash space", as it's a normal dash used for punctuation, sort of a comma or a colon. And the second one is a dash used marking a broken word (burglar) or unfinished sentence, so it should (could) be left as an em-dash, with a space to separate the following text. There could be other cases.
As for the reverse transformation (en-dashes to em-dashes), it's probably safe to replace all "space en-dash space", "space en-dash" and "en-dash space" with an em-dash.