Can I undo the upgrade to iOS 7 by restoring to iOS 6.1.3? I recently attempted to follow this tutorial to upgrade my device (iPhone 4) to iOS 7 without being a developer or registering a UDID. 

I loaded and installed iOS 7 and got to the configuration screen. However efter attempting to go through the configuration process it says:


  We’re unable to complete your activation. This device is not
  registered as part of the iPhone Developer Program. If you are a
  member of the Program, please register your device in the iPhone
  Developer Program Portal.


The problem is this is also what it says on iTunes so there is no way to access it from the computer. 

How can I downgrade back to 6.1.3? I suppose I could register as a paid developer for iOS 7, but that involves forking over $99 and I'm not even sure I could revert to iOS 6 even then.

Additional Information:


I started with an unjailbroken iOS 6.1.3.
I downloaded iOS 7.0 CDMA for iPhone 4 from here
I have Verizon
I Have the iOS 6.1.3 version for my iPhone downloaded from here, but I am unsure how I can restore from it because iTunes will not let me do anything because of the above mentioned message. 


Update: I managed to entere recovery mode by pressing "power" for 3 seconds, "home" and power for 10 seconds, and just "home" until it had entered recovery mode. I have then begun to restore the phone which requires iOS 6.1.3 to download again through iTunes. Note that the first time you restore it may not work. Mine still booted to iOS 7 until I tried again.
It is likely the baseband was updated along with the OS, but if you have a good backup of the phone, I might try restoring it in recovery mode.


Power off the iPhone
Plug in the iPhone to your computer via USB.
Press and hold the "power" button with one hand for 3 seconds
Hold down the "home" button as well for ten more seconds
Release "power" button but continue to hold "home" button until device enters recovery mode.


I don't have a device to test this and it might not work. You might have to get a jailbreak tool to modify the OS and/or firmware of the device and then restore 6.1.3 on it.