For our open search cluster 1 3 AZ's, with a modest 3x30gb EBS gp2 EBS's, changing from gp2 to gp3 took about 23 minutes in total.
After about 15 minutes the new EBS types was probably operational and teardown of the old cluster in the blue-green deployment began.
We experienced no downtime, although longer running connections for updates and so on might probably experience a disconnection during the update.
We got nice status updates from our domain view: