Try to do like this:
Login to your AWS Management Console, in EC2 tab view, you'll find something like 1 key Pair, click it, delete your old key pair, then create new pair.
EDIT:
Got a related question HERE
What you could do, is assign one of your Elastic IP's to the instance and route traffic through that normally. Snapshot the instance and bring up a duplicate with a new Keypair. Switch the Elastic IP over to the new instance.
and HERE
The best solution we have been able to come up with is to burn an AMI from the running instance and launch a new instance from that AMI using the new key pair.
How to create your own AMI? HERE. ;)
