I dunno, maybe I'm just overthinking things, but, in case theymos feels that the old behavior is worth preserving, here are two additional diffs that will make sure (at least, in the two places that are important, I think) that old posts won't be affected by this fix:
Well, if you ask me, that's a bug, and you're fixing it. There shouldn't be any issue implementing this change, and I think it should be fixed in the old posts too. After all, you're just adjusting how it behaves on the front end without affecting the original posts.
On the other hand I guess that's why SMF have
[ code ][/ code ] tag for such things.