Once an instance is launched, you can associate a new or different Elastic IP to the running instance.
WARNING!
Once you select an Elastic IP and click the OK button, the instance will start to receive
traffic for that IP address once the IP address has enough time to transfer over to the new instance (about 5 minutes). Therefore, make sure you are not stealing an Elastic IP from another running instance and that it
is OK to inherit traffic from the previous owner of that Elastic IP.
If you are using an Elastic IP that you know has not been used for over
24 hours, you will probably not inherit old traffic.
You can associate an Elastic IP to a running instance at the deployment and instance levels.
At the deployment level you can click the instance's Edit icon and select the appropriate Elastic IP.

To associate an Elastic IP at the instance level, under the Info tab, click the Edit icon to select an EIP to associate to the running instance. If the instance already has an EIP, you can only choose a different EIP, otherwise click Cancel.