This isn't quite what you want to do ... but hopefully it's helpful.
Go to the class view, and rename the namespace using Ctrl+R,R. It will update that namespace in all the files/folders that it's used in. As long as your namespaces are consistant, it should acheive the same result as changing all the namespaces in a folder.
If your namespaces aren't consistant, and you're just tidying up then I'm afraid you've got a lot of clicking in front of you (or behind you as you've probably already done this).