Your AWS dashboard would be compromised if you waited until you were on the open wireless network to run the EC2 instance, but attackers would not be able to compromise the EC2 instance. Once the instance has been created you can't change the Key Pair. You also can only download the private key associated with the Key Pair once, which is right when you create it. But of course, an attacker could stop or terminate your instance if he gained access to your AWS dashboard.