Its a full upgrade. Its on offer at a price of £0 for the first year. After that buying a Windows 10 upgrade will cost £something.
They have only confirmed you can move to the final release if your running Windows 10 Preview having installed it over a Windows 7 or 8.1 install though the Windows Update method.
MS have not confirmed how (or if) you will be able to do it if you clean installed Windows 10 Preview, as there is no license key anywhere in those installs.
I was under the impression that the license keys would still work that you had before installing the Preview?


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