There is no data that directly speaks to how a change from free to paid will impact the uptake rate of Windows 10 before and after the one-year span, but Windows 8's progress provides a clue. For several months in late 2012-early 2013, Microsoft delivered cheaper upgrades from Windows XP, Vista and Windows 7 to Windows 8 before raising the price five-fold. (The early adopters' price was $40 to Windows 8 Pro; after the discount expired, prices jumped to $200.)
According to analytics vendor Net Applications, which tracks adoption via user share, Windows 8 grew between 0.63 and 0.77 percentage points each month for the three months when the $40 upgrade was available. Once the reduced price vanished, Windows 8's monthly increase in share dropped to 0.47 percentage points before recovering to the mid-.05 point range.
Those numbers are total uptake, not just upgrades, as they account for new system purchases too, but they imply a quick start to Windows 8's climbing share as the $40 upgrades were shipped to customers, then a declining rate of change for the next several months. Expect the same to happen with Windows 10: A big boost to its share at the beginning of the free upgrade, a slowing of the month-over-month increase as time goes on, and a significant drop once the offer expires.
However, Microsoft will more accurately monitor the progress of Windows 10 adoption -- primarily through product activations -- and so could easily extend the free upgrade offer if it believes the deal has not sufficiently moved the needle toward the 1 billion goal.
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