We have been using b2evo since 0.9.2, we are now at 6.10.2. The thing is that in the old versions, posts were saved with no </p>'s, just a blank line. But now tinymce saves all posts with <br /> and </p>. The problem is that when we try to edit an old post, tinymce finds no line or paragraph separators, so it just shows a long text without paragraphs. If you are able to edit the post in markup mode, you still see the blank lines, but when you go to wysiwyg mode they dissapear for good.

So the questions if there is a possibility, before tinymce takes command, to ask $content if there is any </p> (or any other method). I imagine something like:

$parag = stripos($content, '</p>');
if ($parag === false)
{
$content = autop($content);
}
else
{
do nothing;
}