I think OP had a legit question here. The simple solution was the one taken in the end - check a whole bunch of the sites for a consensus on the result. The re-grade is based on the correct result, that said I wouldn't hurry to give Pinnacle back their money, just give it back to them next time you are ready to play there.
I doubt many of the people who say that they miss Pinny were actually beating them, sure you were getting better prices on your bets - but I doubt you were beating them on major sports.