I've been looking all over Xcode for this, but I can't find any place that allows you to rename an existing scheme in Xcode 4. Is this even possible?
You need to click on the scheme button, then hit "Manage Schemes". Once you do so, you'll have a window like this:
Select the scheme you want to change, and hit Return.
Now the scheme name will be editable, and you can change it to your hearts desire. Then hit Return again to save it.