The only way that plan is available is either at WM or order via the web. Policy is that it's available only for new activations, so be prepared simply to activate a new number--but before you do that, call customer service and ask if they'll simply move you to that plan. If you read some threads in here, you'll find that some people were successful with that.
But the store can't do it at all. Screwed up, but that's the way it is.
I use the $30 plan... or at least I did until my service went out. I'm in an excellent coverage zone and gave no service. If the tower is down, where us AT&T? I'm using my ST AT&T service in my 4s and have great coverage.