To force a full refresh of the package database, update all packages on the system and allow packages to be downgraded. Downgrading should be only be needed when switching to an older branch. For example, switching from Testing to Stable.
强制更新整个包数据库，更新系统上的所有软件包，同时允许降级部分软件包。软件包的降级只可能发生于，从较新的分支切换到较老的分支时。（例如：从Testing分支切换到Stable分支）