Are Amazon RDS instances upgradable?

Yes, Amazon RDS instances are upgradeable via the modify-db-instance command. There is no need for data migration.

From the Amazon RDS Documentation:

"If you're unsure how much CPU you need, we recommend starting with the db.m1.small DB Instance class and monitoring CPU utilization with Amazon's CloudWatch service. If your DB Instance is CPU bound, you can easily upgrade to a larger DB Instance class using the rds-modify-db-instance command.

Amazon RDS will perform the upgrade during the next maintenance window. If you want the upgrade to be performed now, rather than waiting for the maintenance window, specify the --apply-immediately option. Warning: changing the DB Instance class requires a brief outage for your DB Instance."

RE: Outage Time: we have a SQL Server 2012 RDS Instance (1TB non IOPS drive), and going from an db.m1.xlarge to db.m3.xlarge (more CPU, less $$) incurred just over 4 minutes of downtime.

NOTE: We did the upgrade from the AWS console GUI and selected "Apply Immediately", but it was 10 minutes before the outage actually began. The RDS status indicated "Modifying" immediately after we initiated the update, and it stayed this way through the wait time and the outage time.

Hope this helps!

Greg