In that case, it simply means dumping Vista and returning to XP. To the vast bulk of users, though, ‘downgrade’ is a synonym for reverting to an older version. By Microsoft’s end-user licensing agreement (EULA), you can’t have both the Vista and its downgraded XP installed at the same time on the same or different machines.

Think of it as a swap, Vista for XP, not as an extra licence. In effect, the licence for Vista is transferred to XP.

Specifically, these downgrade rights let owners of some versions of Vista replace it with XP without having to pay for another licence. In an older-to-newer move, developers usually make it possible to retain all the digital detritus on the drive, from already-installed applications and Word documents to iTunes tracks and family photos, while updating the system files. Downgrade doesn’t mean the process for rolling back Windows from Vista to XP, since there isn’t such a procedure, not in the generally accepted use of ‘upgrade’. To Microsoft, ‘downgrade’ describes the licensing rights it grants to older operating systems.

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