I am sure most of this change is due ti the sales of new devices. Math examples:
500 MM iOS devices in us3
52% are on iOS 8 on Oct 27 = 260 M devices.
They sell 20 M new iOS 8 devices in the last 3 weeks.
Now 280 M out of 520 M on iS 8 = 53.8 %
To get to 56% about 11 M legacy devices had to upgrade.
A fair number of those are schools with fleet upgrades.
So I don't think the remaining base of consumers is upgrading very quickly.
In my family we have 6 iOS...