I'd call again tomorrow. If they claim they refunded you the money, ask them to email you a receipt reflecting that. (Of course, check your card statement first the confirm.) if they didn't do the refund, let them know PS has been restore and you'd like your seat un-downgraded. If they try to say they'll refund you instead, say that's not what you paid for but, at they very least, they should, then and there, let you "buy up" to PS for the same amount as the downgrade compensation. Of course, that would be much more work for them (manual repricing and whatnot) than just restoring you to PS.

If that doesn't work, I'd consider drafting a quick letter to DOT and copying Delta.