Unable to update DNS TTL

Hello!

Currently i am preparing to move couple of servers. This move will incorporate change IP addresses of servers. Since i am hosting my DNS with Mail-in-a-Box, i would like to lower TTL of DNS records, so clients wouldn’t have big downtime.

I’ve tried to update management/dns_update.py:

grep TTL management/dns_update.py
        # the TLS certificate used by Postfix for STARTTLS must be a valid certificate for the MX
        # For the refresh through TTL fields, a good reference is:
$TTL 300          ; default time to live
           300      ; Negative TTL (how long negative responses are cached)

and later restart mailinabox service(and fully restart server itself), but neither sudo tools/dns_update --force nor adding stub record and removing it later doesn’t help with change of TTL of my record.

How can i change TTL of my DNS records?