The switch to Bb/F horns happened in 2000. According to http://www.dci.org/news/view.cfm?news_id=ca77ecfc-38f5-4c64-8774-c3c9b5cfc9cc , the cadets and BD were the first 2 corps to switch away from G. The last corps to switch to Bb/F was the Racine Scouts, and they marched their season away form G bugles in 2011, according to
I'm not sure why DCI made the shift, but a big part of it was about evolving the activity and making it on the same page and more available for everybody to participate in. This way I can bring my B flat trumpet to a camp and play that, instead of getting used to a new instrument (same basic concept, sure, but they handle differently from what I've heard). I don't know much more than this, sorry.