Some of the packages were obsoleted between CentOS 6 and CentOS 7. This means
CentOS provides some alternative for them, but these alternatives may be
not 100% compatible - thus we don't replace them automatically. For some of
the obsoletes you will get the incompatibilities from separate preupgrade
contents and you can adjust your workflow according to the changes. Sometimes,
package might be replaced by several others.

Following packages were obsoleted by different ones:
ConsoleKit (required by NonCentOS signed package(s):libfprint ) was obsoleted by systemd
PackageKit-gtk-module was obsoleted by PackageKit-gtk3-module
cpuspeed was obsoleted by kernel-tools,kernel
gdm-user-switch-applet was obsoleted by gnome-shell
gnome-applets was obsoleted by gnome-classic-session
grub was obsoleted by grub2
gvfs-obexftp (required by NonCentOS signed package(s):gnome-bluetooth ) was obsoleted by gnome-bluetooth
hal (required by NonCentOS signed package(s):libgpod libmtp system-setup-keyboard gnome-power-manager ) was obsoleted by systemd
mysql-libs was obsoleted by mariadb-libs
mysql-server was obsoleted by mariadb-server
mysql was obsoleted by mariadb
nc was obsoleted by nmap-ncat
pam_passwdqc was obsoleted by libpwquality
polkit-gnome was obsoleted by gnome-shell
udev (required by NonCentOS signed package(s):DeviceKit-power libmtp media-player-info openct pcmciautils iwl5150-firmware iwl100-firmware iwl6050-firmware iwl6000g2a-firmware aic94xx-firmware iwl6000-firmware iwl1000-firmware ) was obsoleted by systemd
udisks was obsoleted by udisks2
upstart (required by NonCentOS signed package(s):readahead ) was obsoleted by systemd

If some NonCentOS signed package requires these packages, you may need to check if the
alternative solution provided by CentOS does work for you or get the missing package
from different sources than CentOS. You need to install these new packages yourself
after the assessment, as CentOS can't assess the compatibility for you.