In general, for each shard: More detailed:

Meaning - You won't be going to have read downtime at all if you have read from a replica.
You will write downtime for single shard each time of few seconds. Generally, the downtime is when the new and old masters replace the DNS, so it's the time it takes to replace the DNS.

Migrating to Valkey 8?
Resource: https://docs.aws.amazon.com/AmazonElastiCache/latest/dg/VersionManagement.html