yum update vs yum upgrade

$ man yum

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update

If  run  without  any  packages, update will update every currently installed package.  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. (See Specifying package names for more information) If the packages or globs specified match to packages which are not currently installed then update will not install them.  update  operates 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 noticeable 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.

upgrade

Is the same as the update command with the --obsoletes flag set. See update for more details.

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https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/System_Administrators_Guide/sec-Configuring_Yum_and_Yum_Repositories.html

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obsoletes=value

The obsoletes option enables the obsoletes process logic during updates.When one package declares in its spec file that it obsoletes another package, the latter package is replaced by the former package when the former package is installed. Obsoletes are declared, for example, when a package is renamed. Replace value with one of:

0 — Disable yum's obsoletes processing logic when performing updates.

1 (default) — Enable yum's obsoletes processing logic when performing updates.

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