I switched back to my iPhone 4 just to try out iOS 7 and it isn't that great and i want to switch back but instead i get this
Your Device Change request failed and cannot be completed online. Please contact Customer Service at 1.800.922.0204
Whats going on why has it failed
I imagine the only one's who can tell you that, would be customer service. Not sure what phone you're switching back to, or from, but some (mainly 4G/LTE ones) may require a new SIM or something along those lines. I'd give them a call, as that seems the only way you'll get to the bottom of it.