Fix some terminology and drop an unneeded if

There were some places where the "updater" terminology was still in
use; those have now been changed over to "upgrader" to correspond to
the upcoming "downgrader".  There was also an unnecessary 'if'
statement preceding a loop; that has been removed.
Add the concept of downgraders

This adds downgraders, which take a state of one version and downgrade
it to the format of an older version.  This just adds the concept;
no code is currently using the downgrader.