I usually go with 300, as that is 5 minutes, and set that up a few days before I plan to do a migration. That way the lowered TTL has time to propagate before I make any changes.
To change the default value, you'd want to go to the very first option in WHM, Basic WebHost Manager setup, and change the value next to this text to 300:
"This value defines the caching time for host name lookups for domains hosted on this server. When a caching nameserver queries the authoritative nameserver for a resource record, this value defines the number of seconds before the record will expire. Shorter TTL values can cause heavier loads on the nameserver, but can be useful for zones that contain frequently changing records. The range of values in seconds as defined by RFC 2181 is 0 to 2147483647."
That will change the value for any newly-created zones.