From our Apple Q1 2015 transcript:
At this point in the — just shortly after the quarter — we don't have all of our research in from all of the people that bought [iPhones] last quarter yet, but in the aggregate, as I've mentioned before, we saw more new customers to iPhone than we had ever seen before, and we had a higher rate of Android switchers than we had in the three previous launches, and it's not that we had more in the fourth one — I don't know what those numbers are, it wasn't something we were looking at.
And so between the switchers and the people that are just new to smartphones and selected an iPhone, and our upgrades — which we're very happy with but represent a small fraction of our install base — we feel really great about what's in front of us.
Talk about understatements... In a later interview, Cook elaborated that only 15% of new iPhone purchases were from shoppers upgrading from a previous iPhone.