I downgraded my prepaid plan today, now I have no service and the site is demanding a payment to start my "new service."

When you change your plan, the old plan terminates and you lose any remaining days. You don’t get credit for those last 5 days. The new plan doesn’t start until you pay for it. That is the way prepaid has always worked. You should have waited until the last day of your unlimited plan to change. Yes, you can wait until the 16th to renew your plan but you won’t have service until you pay for your new plan.  No, there is no way to revert back to your old plan since it was terminated. 

Thanks @sandblaster!  It's good to know that my account and phone number will still be there when I re-fund the new plan in five days, but saying "that's the way it's always been" doesn't make this practice any less disingenuous. I had credit remaining on my old plan, but AT&T terminated my service and kept my credit for themselves without really making it clear that that's what was happening.

For any other interested parties, here's what AT&T's polite plan-change disclaimer is really saying (and what it doesn't say.)

"When you click 'OK,' your current service will be terminated immediately. Unused services will not carry over, you will not be credited or refunded for any remaining airtime or data on your current plan, and you will be required to prepay for your new plan before your wireless service is restored.

For a more painless transition, turn off the automatic payment on your current plan, purchase your new plan the day after the old plan expires, and then turn autopay back on."