There is a very elegant way to update your DNS if you are running djbdns: You can optionally specify date ranges for every record![1] The server will automatically adjust the TTL. By having two records with different time ranges you can switch IP addresses at an exact moment.

The timestamps are provided in DJBs TAI64 format, use something like https://github.com/hinnerk/py-tai64 to convert them.

[1] config file spec at https://cr.yp.to/djbdns/tinydns-data.html