I am in the process of server upgrades. Going from i7-2600 platforms to i7-6700k platforms with RAID 10 arrays for home directories and mail spool.
The host operating system is being switched from Centos 6 to Ubuntu 15.04. The primary motivation being RedHat is terrible with keeping things current, even with CentOS 7, and there are some substantial capabilities and performance improvements to be realized with modern software.
Ubuntu also provides a means of upgrading major releases in place rather than requiring a full re-install as is the case with Centos and this will make it easier to stay current.
Lastly, Ubuntu features a very good software installation / upgrade package (synaptic) while CentOS is going from bad to worse with their yum based package installer. Given the direction Fedora has gone (dnf) I want to bail before that becomes part of CentOS.
This will require some downtime of various hosts as they are moved from one machine to another but it should be minimal and will be done during low traffic hours.