====== update と upgrade の違い  ======

===== 結論  =====
同じ。

yum upgrade は yum update --obsoletes のことだが、obsoletes オプションは標準では true なので、yum update でも yum update --obsoletes になる。
===== man yum  =====
<code>
       upgrade
             Is  the  same as the update command with the --obsoletes flag set. See update for
             more details.
</code>

<code>
       --obsoletes
             This option only has affect for an update, it enables yum´s obsoletes  processing
             logic. For more information see the update command above.
             Configuration Option: obsoletes
</code>

<code>
       update If run without any packages, update will update every currently  installed  pack-
             age.   If  one  or  more  packages  or package globs are specified, Yum will only
             update the listed packages.  While updating packages, yum will  ensure  that  all
             dependencies  are satisfied. If the packages or globs specified match to packages
             which are not currently installed then update will not install them. update oper-
             ates on groups, files, provides and filelists just like the "install" command.

             If  the main obsoletes configure option is true (default) or the --obsoletes flag
             is present yum will include package obsoletes in its calculations - this makes it
             better  for  distro-version changes, for example: upgrading from somelinux 8.0 to
             somelinux 9.

             Note that "update" works on installed packages first, and only if  there  are  no
             matches  does  it  look  for available packages. The difference is most noticable
             when you do "update foo-1-2" which will act exactly as "update foo" if foo-1-2 is
             installed. You can use the "update-to" if you’d prefer that nothing happen in the
             above case.
</code>

obsoletes：旧式パッケージになっているパッケージを削除され、適切なものにアップデートされる。