> Having no-down-time when doing migrations is not an easy topic.

Depending on the nature of the migration and the type of data and the type of service and how coupled everything is, it's impossible. Especially when you are dealing with financial data and transactions that you can't afford to screw up even a single one.

> I usually vote for having some planned non-working-hours downtime.

Yep. No other way around it. You tell your clients weeks/months ahead of the planned downtime. They'll understand and migrate in the middle of the night over a holiday weekend.

But this is in regards to major structural migrations, not simple schema updates or changes.