The major differences between YUM and RPM are that yum knows how to resolve dependencies and can source these additional packages when doing its work. Though rpm can alert you to these dependencies, it is unable to source additional packages.
As to installing vs. upgrading. Both tools can perform an install, and RPM will even allow you to install multiple versions simultaneously, but YUM will tell you that that package is already installed. So no YUM will not allow you to install multiple versions.
As to yum update, this will react in one of two ways. If you tell it a package that you want to update, it will attempt to do so, downloading all the necessary dependencies and installing them too. If you run it without any package name, yum update will attempt to update every package that's installed on your system.
If you use yum upgrade it will do the same as yum update, except it will attempt to remove any packages that have been marked as "obsolete".
Example
Here's a scenario not unlike your own. A new version of vim-X11 is available.
$ yum check-update vim-X11 | expand
Loaded plugins: auto-update-debuginfo, changelog, langpacks, refresh-packagekit
vim-X11.x86_64 2:7.4.417-1.fc19 updates
And I'm currently at this version:
$ rpm -q vim-X11 | expand
vim-X11-7.4.179-1.fc19.x86_64
When we try to install it:
$ sudo yum install vim-X11
...
=================================================================================================================================================================
Package Arch Version Repository Size
=================================================================================================================================================================
Updating:
vim-X11 x86_64 2:7.4.417-1.fc19 updates 1.2 M
Updating for dependencies:
vim-common x86_64 2:7.4.417-1.fc19 updates 5.9 M
vim-enhanced x86_64 2:7.4.417-1.fc19 updates 1.0 M
....
Running transaction
Updating : 2:vim-common-7.4.417-1.fc19.x86_64 1/6
Updating : 2:vim-enhanced-7.4.417-1.fc19.x86_64 2/6
Updating : 2:vim-X11-7.4.417-1.fc19.x86_64 3/6
Cleanup : 2:vim-X11-7.4.179-1.fc19.x86_64 4/6
Cleanup : 2:vim-enhanced-7.4.179-1.fc19.x86_64 5/6
Cleanup : 2:vim-common-7.4.179-1.fc19.x86_64 6/6
Verifying : 2:vim-enhanced-7.4.417-1.fc19.x86_64 1/6
Verifying : 2:vim-X11-7.4.417-1.fc19.x86_64 2/6
Verifying : 2:vim-common-7.4.417-1.fc19.x86_64 3/6
Verifying : 2:vim-enhanced-7.4.179-1.fc19.x86_64 4/6
Verifying : 2:vim-X11-7.4.179-1.fc19.x86_64 5/6
Verifying : 2:vim-common-7.4.179-1.fc19.x86_64 6/6
So in summary
- Will performing a
yum install on 1.1 after 1.0 is already installed, list 2 packages?
No. YUM will still perform an update even when you've told it to do an install if the package is already installed.
- Will running
yum update some-package-1.1 successfully replace some-package-1.0 ?
Yes.
As mentioned in the previous question, CentOS is your best choice since it is derived from the sources of Red Hat Enterprise Linux (RHEL).
Also as mentioned in their Technical and Release notes, almost all your required tasks are met in CentOS.
Best Answer
https://apps.fedoraproject.org/packages/yum-plugin-merge-conf