If you purchased an annual licnese via the Amazon Marketplace, do not change the instance size after installation. Doing so will switch your instance to become hourly billed.
So if you ever plan to up/downscale without hassle on the FileMaker Cloud servers, you better go and buy the license from FileMaker directly (or a your favorite reseller).
With that license, you can easily switch from t2.medium on development (2 cores, 4 GB RAM) to t2.large (2 cores, 8 RAM) for a beta test and up to t2.2xlarge (8 cores and 32 GB RAM) for deployment on busy days. It would be pity if your next AWS bill for FIleMaker Cloud would be $700 for hourly billing, because you just changed instance type.
And you can use one license over the year for several servers independently. e.g. buy a FileMaker Server for a project in February, terminate the server in April and later in August start a new one for another project with the existing license.
For details, please check answer 25984.