How long have you been on pre-paid? Many times within a month, an account change can be undone if you plea a case with Customer Service and submit a request for inactive pricing. The Unlimited Data codes can still be provisioned but the agents are strongly recommended against doing that. I believe the request for inactive pricing may also only apply for those who get a subsidized phone versus a phone on the device payment plan, and then suffer from buyer's remorse. But it's a worthwhile try if you're willing to go back to Postpaid.
But RIP to that unlimited data. That is the most valuable phone plan in the US at the moment because you can't buy it, they are becoming increasingly rare, and there's no throttling or worry about data caps due to both the plan, and FCC Regulation. AT&T, Sprint and T-Mobile sell unlimited data... but there's de-prioritization, and with AT&T you're paying a mad amount of money for DirecTV / U-Verse TV + the per-line unlimited data fee.