Making html work in posts

Some of our threads when we were on vBulletin were written in html and now that we're on Xenforo, we're left with broken html code that doesn't work.

How could we go about fixing this?

 

I don't know if there is a mod, according to this thread you might check
HTML BBCode (permission protected)
or
Kier's content find and replace mod
Chris Deeming said you'll need some knowledge of regular expressions, that you can mess things up if you're not exact.