I would like to apply server hardening policy[securing] on existing Ubuntu production servers. Will lead any issues after applying? What is the best practices to apply hardening on existing production environments.
Best practices: Understand what the production servers do (deep internals), and consider whether or not your "hardening" will interfere with what they do.
"Harden" your test servers first.
A tool that I like is
lynis from cisofy.com. It is free, and makes no changes, just points to vulnerabilities.
Realize that security/hardening is a process, not a paint. You cannot just spay it on once (and succeed).