Steps to Encrypt Volumes

Here the steps to encrypt an unencrypted volume.

Take a snapshot of the unencrypted volume.
Make a copy of that snapshot and turn on encryption.
Create a volume of the encrypted snapshot.
Stop the instance.
Detach the original unencrypted volume from the instance.
Attach the newly created encrypted volume to the instance.
Start the instance.

AWS CLI

# CREATE A SNAPSHOT
aws ec2 create-snapshot \
--volume-id vol-1234567890abcdef0 \
--description "This is my snapshot"
 
# COPY SNAPSHOT
aws ec2 copy-snapshot \
--source-region us-west-2 --source-snapshot-id snap-066877671789bd71b \
--region us-east-1 --description "This is my copied snapshot."
 
# CREATE A VOLUME
aws ec2 create-volume \
--region us-east-1 --availability-zone us-east-1a \
--snapshot-id snap-066877671789bd71b --volume-type io1 --iops 1000
 
# STOP AN INSTANCE
aws ec2 stop-instances --instance-ids i-1234567890abcdef0
 
# DETACH A VOLUME
aws ec2 detach-volume --volume-id vol-1234567890abcdef0
 
# ATTACH A VOLUME
aws ec2 attach-volume --volume-id vol-1234567890abcdef0 \
--instance-id i-01474ef662b89480 --device /dev/sdf
 
# START AN INSTANCE
aws ec2 start-instances --instance-ids i-1234567890abcdef0