= Problems with performing downgrades =

There are a number of problems with performing downgrades. I'll cover a few of them here:

 * scriptlets are not reversible:
   rpm scriptlets provide no way to reverse the process(es) they executed. For example a scriptlet (%pre, %preun, %post, %postun, etc)
   may run echo "Foo foo foo" >> /etc/some/file. There's no way to undo that when you perform a downgrade.

 * downgrading works provided the user data/user config is not modified by
   an update in a one-way process. 
    Examples: mysql upgrade from  4->5 will convert a db, but going back the other way won't fly
              upgrading a desktop application from one version to the next major version modifies config files in such a way but cannot
              modify them back.

 * There are certain processes which no one is ever going to do the work
   to make them reversible: lvm1->lvm2, db transitions, udev migration,
   ext3->ext4.

 * Currently the depsolver doesn't work very well for downgrades, as it can't downgrade extra packages. However you can use tools like yum-debug-dump and yum-debug-restore, which should include all packages needed.

There is now a downgrade command.