This a light-weight add-on for changing shipping/billing address directly in Checkout Confirm page.
If you use checkout payment modules like PayPal Express checkout, which obtain address from PayPal website, or using one page checkout, they would skip Checkout Shipping page and bring customers into Checkout Confirm page direct. Customers who want to change default address have to jump backward to Checkout Shipping page and forward causing bad user experience. In this case Ajax can greatly help and allow customers staying in same page with less clicks. A alert would appear if customers didn't confirm the address before confirming the order.
Note: It would only change the address of the orders, not the address in customers' address books.
Support forum: http://forums.oscommerce.com/topic/394493-addon-ajaxcheckoutaddress/