With the introduction of Elastic EBS volumes, it is now possible to expand the size of EBS volumes, change the volume type between General Purpose SSD (gp2) and Provisioned IOPS SSD (io1), and adjust the IOPS performance for an io1 volume.
For current generation instances(*m3) this can be done while the instance is running, and so means that instances no longer have to be replaced in order to increase disk space or change volume type. For older instance types, changes to volumes can only be made whilst the instance is powered down, and thus it is recommended in the majority of such cases to boot an application or database slave instance with the required volumes and promote this application or database slave in order to replace the existing instance and volumes.
*m3 - for m3 instance types only the m3.medium is considered "current generation", the other m3 instance types are dependent on the instance age (newer ones usually support it).
It should be noted that volumes can only be expanded. If you require to decrease the size of a volume, you must boot a new instance with a smaller new volume, without the use of a snapshot. For database instances this will therefore require the performance of a database dump and restore. For volume type changes and IOPS increases and decreases there are no limitations other than that there must be a 6 hour delay between volume modifications.
Currently any Elastic EBS modifications are required to be carried out by Engine Yard Support Engineers, therefore please submit a ticket to request any modifications.