Instance started from PV AMI's cannot be modified to new generation instance types

If you have long running instances from PV type of AMI, then when you encounter an instance retirement, you cannot modify the instance type to m3.* or c3.* because Amazon would like everyone to move to HVM based images. For more information regarding PV and HVM virtualizations, you can refer to this link

For now, if you have PV virtualization instance, then you can only modify the instance type to first generation instances in us-east-1:-

Surprisingly, m1.large is missing from the above list and the reason for that is that ami that was created from the running instance is a 32 bit instance. In order to migrate to an m1.large, the original instance would need to be migrated to a 64 bit instance first.