How does the replication service handle schema changes?

Logical replication allows for some schema differences between the publisher and the subscriber, but in general, the less you change your schema, the better. Best is if you don't change your schema at all.

On our end, when we make schema changes, we will give you advance notice of any breaking changes and we will complete the work according to a pre-determined maintenance schedule.

In practice, breaking changes are quite rare so schema migrations are largely additive.