To change the storage class on all objects within a bucket use:
aws s3 cp s3://<bucket-name>/ s3://<bucket-name>/ --recursive --storage-class <storage_class>
There is another way to do this.
Using s3cmd:
s3cmd cp s3://BUCKET/KEY s3://BUCKET/KEY --storage-class=STANDARD_IA
Using AWS CLI:
aws s3 cp s3://BUCKET/KEY s3://BUCKET/KEY --storage-class STANDARD_IA
I hope this helps you.