Those that wanted something new and exciting left BB for Android and iOS. Those that stayed are either in 3rd world countries where BB handsets were cheap or loyal users to BB in US and Europe who didn't need a modern smartphone.
Those in 3rd would countries will by and large not upgrade to BB 10, they don't have the money.
My belief the number could be as low as 10% of users upgrading and as high as 20% if BB does an effective campaign to advertise.
You think the countries that have 1/10th the GDP per Capita are going to rush out and buy a expensive handset? I don't, no one who knows math does. THOSE people make a huge part of the current user base.
Now here is what RIMM needs to do. They need to be working on a product line that is similar to Nokia's Asha line, cheap and affordable but at the same time a quality product. BB 10 just will not cut it, not enough interest in the markets that can actually afford to upgrade their handset.
What about those returning...I myself left BB for iphone 4 back in September 2010. I now have 4s and will be going back to BB for Z10. I left because I needed an all touch with a great browser...I believe (and have heard from direct sources) that BB10 will have best browser on market...OS is better than iOS and Android.
iphone is boring now to me and Samsungs are junk in my opinion (my mom, sisters and sister-in-laws all have them and I see first hand that they are junk)
BB10 will surprise a lot of people and the timing is actually perfect...BB will regain market share in the US, UK (Euro) and Canada...watch and see
Shorts be warned and best of luck to all in the New Year