When creating an image (EC2 --> Actions --> Image --> Create) there is a Volume Type input with options for Magnetic, General Purpose or Provisioned IOPS. I'm not sure why it's necessary to ask this here since I'd thought that, when it's time to create an instance from this launched AMI, I could then specify the volume type that the instance is reconstituted upon. So my question is, do I need to exercise care here or can I modify the volume later?
The volume type you specify for the AMI becomes the default volume type for launches. The EC2 console does make it easy to select what volume type you want when you launch, but support for that varies across client tools.
The Amazon Linux AMI makes their EBS-backed AMIs available for both Magnetic and General Purpose SSD by default:
$ aws ec2 describe-images --owner amazon | grep '2014.09.1 x86_64 HVM' | cut -f 3,5 Amazon Linux AMI 2014.09.1 x86_64 HVM GP2 ami-b1a7ea81 Amazon Linux AMI 2014.09.1 x86_64 HVM EBS ami-b5a7ea85