Sgt_Kickaxe - 5:26 pm on Oct 28, 2011 (gmt 0)
Once fixed you will have TWO sets of urls and the "fixed" version will start to gain history. If you 301 the old to the new and/or robots.txt block the old, and possibly even perform a removal request if you take the robots.txt route, you can be assured that the new will outrank the old.
That being said the old url's will need a good long time to fall out of your link graph, even if they don't rank in serps. Google forgets nothing.