There might be other way(s).
I don't know enough about CentOS7 and Zimbra to guide you.
I can only say, be sure to backup your system regularly - especially before such an upgrade.
ok thank you very much for the advice
There might be many ways to Rome, but it's good practice to keep your system healthy by updating the software regularly. Not only to get the latest features of the software, but also to keep it up to date with security updates. Your software might be vulnerable to remote exploits otherwise (although there are always chances of zero-day exploits).