It’s 2017 and it’s still not possible to reliably upgrade an Enterprise Linux distribution between major versions!
At this point you are scrolling down to the comments to “educate me” about the redhat-upgrade-tool, because you read about it somewhere and you once heard someone successfully upgraded an installation on a lab machine. Unless you have an ultra vanilla starting point, you are going to end up with a mess that probably won’t boot. By the time you come to upgrade a “real server”, there have been years of changes and it is unlikely to resemble some pristine minimal installation.
I know your next comment is going to be something about the architectural changes brought in by project X and version Y in RHEL7, which is why it is all so hard. Stop now! You are boring me already! Is it an enterprise distribution or isn’t it? If it is, you should be able to upgrade it reliably!
Next up comes, “But you have to reinstall when you get new hardware!” I present to you virtual machines. Physical hardware upgrades with no need to reinstall the OS on the VM.
I can’t believe I’ve been using Linux for about 19 years and this is where we are at.