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In computing, downgrading refers to reverting software (or hardware) back to an older version; downgrade is the opposite of upgrade. Programs may need to be downgraded to remove introduced bugs, restore useful removed features, and to increase speed and/or ease of use. The same can occur with machinery.
The term "downgrade" became especially popularized during the days of Windows Vista, with users wanting to return to, or downgrade to (with some even calling it an "upgrade") Windows XP because Vista had performance and familiarity issues.
Another reason could be that the user's applications do not support their new OS and they want to revert to an older version.
- InfoWorld Bursting the Vista sales bubble: metrics show 35% of enterprise-class users downgrade from Vista to XP, Retrieved on 2009-12-20