# cat /etc/redhat-release 
> Red Hat Enterprise Linux Server release 6.9 Beta (Santiago)

# rpm -q rpm
> rpm-4.8.0-55.el6.x86_64

# yum install -y pcs
> [...]

# rpm -q pcs
> pcs-0.9.155-1.el6.x86_64

#  yum downgrade -y pcs-0.9.90-1.el6_4.noarch.rpm
> [...]

# rpm -q pcs
> pcs-0.9.90-1.el6_4.noarch

# yum --rpmverbosity=debug -d10 -v update pcs-0.9.123-9.el6.x86_64.rpm                                                                                       
> Not loading "rhnplugin" plugin, as it is disabled
> Loading "product-id" plugin
> Loading "search-disabled-repos" plugin
> Loading "security" plugin
> Loading "subscription-manager" plugin
> Updating Subscription Management repositories.
> Unable to read consumer identity
> This system is not registered with an entitlement server. You can use subscription-manager to register.
> Config time: 0.187
> Yum Version: 3.2.29
> rpmdb time: 0.000
> Setting up Update Process
> Examining pcs-0.9.123-9.el6.x86_64.rpm: pcs-0.9.123-9.el6.x86_64
> Marking pcs-0.9.123-9.el6.x86_64.rpm as an update to pcs-0.9.90-1.el6_4.noarch
> Setting up Package Sacks
> pkgsack time: 0.098
> Obs Init time: 0.339
> Building updates object
> up:simple updates time: 0.026
> up:obs time: 0.006
> up:condense time: 0.000
> updates time: 0.244
> No Packages marked for Update

Why hasn't anything been performed?

The same outcome is with:

# yum update-to pcs-0.9.123-9.el6.x86_64.rpm
# yum '--disablerepo=*' {update|update-to} pcs-0.9.123-9.el6.x86_64.rpm

This is contrary to using plain rpm, which just works:

# rpm -Uv pcs-0.9.123-9.el6.x86_64.rpm 
> Preparing packages for installation...
> pcs-0.9.123-9.el6

# rpm -q pcs
> pcs-0.9.123-9.el6.x86_64

So it looks something strange is going on here.

This looks similar to [bug 1337766], but that one fails on unability
to resolve dependencies, so I am unsure if it's just a different
symptom of the same bug, or the cause is different.