See Phil@AWS' reply at https://forums.aws.amazon.com/thread.jspa?messageID=900730󛹺TT
So a big part of answering your question would be if it is a legacy volume or not. For legacy volumes the data is copied by software running in your instance in the background while you continue to use the instance. So this operation is highly variable as it competes with whatever application I/O is going on, limits on your io1 (provisioned) or gp2 (baseline) IOPS and throughput, instance type characteristics, etc. Modern volumes use the EBS Elastic Volume capabilities to make the change, so from an instance-level perspective the switch occurs in a few minutes independent of volume size. The new volume in the new storage type will be in the modern geometry.
When a brief pause is mentioned I believe it means seconds not minutes. The pause is only required in certain circumstances.
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