This happened to me a few months back. My debit card expired and I was issued a new one with a new number.
You'll be notified via email that the charges couldn't be made and they'll ask you to re-submit the correct information. But, when you do that, you'll no longer see the "Unlimited" option when you go to renew.
When this happened to me, I called customer service within the hour (have all your info ready, including the iPad's "cellular number") and explained my situation, saying I had received an updated debit card with a new number, but didn't change all my information in time of the auto-renewal... and that the option for the unlimited plan was no longer available. The rep was more than happy to acknowledge that I was a legacy subscriber and put me back on the unlimited plan.
I hope this works out for you