This. For example, if you have a DNS entry for your DB and the TTL is set to 1 hour, an hour before you intend to make the changes, just lower the TTL of the record to a minute. This allows all clients to be told to only cache for a minute and to do lookups every minute. Then after an hour, make the necessary changes to the record. Within a minute of the changes, the clients should all be using the new record. Once you've confirmed that everything is good, you can then raise TTL to 1 hour again.

This approach does require some more planning and two or three updates to DNS, but minimizes downtime. The reason you may need to keep TTL high is if you have thousands of clients and you know the DNS won't be updated often. Since most providers charge per thousand or million lookups, that adds up quickly when you have thousands of clients who would be doing unnecessary lookups often. Also a larger TTL would minimize the impact of a loss of DNS servers.