Assign EC2 Address
Once you have requested one or more EC2 static IP addresses (on the EC2 Static IP Addresses page), one can be assigned to an existing instance on this page. Simple select it from the IP address to assign menu and click the button at the bottom of the form. This may take several minutes to fully apply, due to the need to adjust routes within the Amazon network. During this time, the system may not be externally accessible on either the old or new address.

If the selected system already has a static address assigned, it can be removed by selecting the None option from the menu. EC2 will select a new dynamic address for the system (which may not be the same as it's original address), and assign it within a few minutes.