Downtime for document DB increase

Hi,

this depends if you have an Amazon Document DB replica instance, in this case the instance change triggers a failover to the replica. This should not take longer than 30 seconds, see https://docs.aws.amazon.com/documentdb/latest/developerguide/failover.html#failover-what_happens

If you have an Amazon DocumentDB replica instance in the same or different Availability Zone when failing over: Amazon DocumentDB flips the canonical name record (CNAME) for your instance to point at the healthy replica, which is, in turn, promoted to become the new primary. Failover typically completes within 30 seconds from start to finish.

Otherwise, it will take a few minutes to create the new instance, see https://docs.aws.amazon.com/documentdb/latest/developerguide/db-instance-modify.html