We were able to workout what is needed.
Essentially the mysql user 'gateway' needs the ability to create and work with the database ssg_testUpdate' as well as the existing 'ssg' database.
Fortunately the AWS admin user we had access to had the ability to update the permissions for 'gateway' user :
grant all privileges on `ssg_testUpgrade`.* to 'gateway'@'%'
Once we had done that the database upgrade went through smoothly.
(note backslashes on the `ssg_testUpgrade` were needed, and a wildcard `ssg%` would probably have worked as well)