[Support Guide] How do I migrate a domain to Netlify DNS with zero downtime?

Good point, @gregraven. DNS changes are not instantaneous. If you are making changes to an existing configuration, we have another guide for how to make those changes with minimal downtime:

This topic (not the one linked above) discusses copying an existing configuration only - not making changes during the move. The recommendation is to have identical (or functionally equivalent) configurations at the previous DNS service and Managed DNS during the migration.

If both configurations are identical, there will be no interruption of service during the time window the changes were occurring. Regardless of whether visitors were getting cached DNS responses from the old service or answers from new Managed DNS service - they should still be getting the same DNS responses.