I know it was possible in iOS 6 to change the carrier name (or logo) displayed in the status-bar.
Is there any way to do so in iOS 7, without jailbreaking the phone?
You can use Carrier Editor
CarrierEditor allows you to customise your iOS device’s carrier logo
Works on Mac OS X 10.6 or greater on any iOS without a jailbreak, but limited to devices that support a carrier.
CarrierEditor generates a modified version of a carrier update onto your desktop and you simply use iTunes to install it.