Like I said, make it convenient, inexpensive, and worthwhile and people will upgrade in droves:
http://www.macstories.net/news/apple-iphone-5-sales-top-five-million-over-opening-weekend-100-million-ios-devices-updated-to-ios-6/
http://www.macstories.net/news/apple-iphone-5-sales-top-five-million-over-opening-weekend-100-million-ios-devices-updated-to-ios-6/