If you are managing a server and are not sure if you should do a new install of the operating system or just upgrade it. You should just do a new install of the operating system. If you decide to do an upgrade, it can leave old files and cause problems later down the road.
When its time to upgrade to a new operating system
You should just do a complete new reinstall. A new install is a fresh start and won’t leave your system with old files from the previous operating system. A new install will also help prevent any conflicts from old settings versus new settings for various programs. Any changes from one version of a program for one version of CentOS or Debian may vary with the latest version. If the latest version of the program uses different config settings or other files and your system has the old files from the previous operating system. It can result in errors and would take you sometime to identify and correct the issue. A new install can prevent this and other issues.
Security issues can be wiped out
If your server was compromised or had any previous security issues that you weren’t aware of. A new install to the new operating system version can help wipe the security issue away. When you don’t do a new install and just an upgrade. Old files will be left on your server and if these files have any security issues. They would continue to exist and be a security threat to your server.