To protect your agile team from scope creep issues, you still need to document changes and get signoff. One way of doing this (if your contract allows) is to get the business sponsor to sign your iteration plan and reprioritized feature/user story list at the start of each iteration. Another way is to write a traditional change authorization form and in it document the specific features/user stories you are being authorized to implement in the current iteration. You may also include in this formal change authorization the adjusted "big picture" release plan.