This is part of our antipatterns series. Ouch!
If you have
yum update in your userdata, however, it updates
ecs-init and that, in turn, will auto-upgrade you to 1.8.0—rolling back to an older image won't help!
yum update --exclude=ecs-init
Best of both worlds: you get the latest security patches and you can roll back to whatever agent you want!
Confirmed to work with Julien of AWS Support. Thanks Julien!