Does implementing X-Forwarded-For irule requires downtime?

I assume you are doing SSL termination(offloading) at the BigIP and want a header to send to your servers to indicate the original request was HTTPS. What we do for this is use a policy on the HTTPS virtual server to insert this header. Attaching the policy will cause no downtime.

ltm policy X-Forwarded-Proto {
    requires { client-ssl http }
    rules {
        X-Forwarded-Proto_insert {
            actions {
                0 {
                    http-header
                    insert
                    name X-Forwarded-Proto
                    value https
                }
            }
            ordinal 1
        }
    }
    strategy first-match
}