I doubt your efforts would be fruitful.
You can discuss the details of your claim with an attorney, but even if they made a mistake, I doubt a lawyer is going to take a $300 case.
Don't see a claim there. If your upgrade is not valid, why would you think you don't have to pay full price? Did you obtain a written guarantee the phone would be in stock and your upgrade would work? Doubt it.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.