Do the new line breaks have to stay?
I realize that, in general, it's a good idea for line breaks to be correctly interpreted as BR tags.
Unfortunately, that's not how it has worked for years, so there are hundreds—hell, probably thousands—of posts that have been hard-coded with BR tags to achieve the desired effect.
As a result of the recent change in the stylesheet or interpreter or whatever, all those BR tags now appear as double BRs—one for the line break and one for the BR tag.
You can't really think that was a good idea.
Either the new interpretation needs to go away, or you need to make it apply only to posts after the date the new interpretation was applied, or you need to offer a simple toggle to turn it off.
What are the chances of that happening?