Question about delayed flights -

My husband and I were schedule to fly back to Charlotte from Helsinki through Dublin last week and we received a notification the morning of our early flight that the plane from DUB was having mechanical issues and the flight would be delayed and we were offered to change our flights. I chatted with an actual agent who changed our flights to HEL to LHR to CLT on the same day but when we tried to print new boarding passes at HEL they did not work and the Finnair agent said the tickets were not valid. We talked to an AA agent and he talked to the Finnair agent and said if we wanted to fly we had to pay $992. At that point I assumed our previous boarding passes were no longer valid and I thought this must be a mistake because the person I chatted with did not tell me there would be a charge. I assumed AA would fix this. (I am not am experienced traveler.) I have contacted customer service and they consider this to be a voluntary change and what I paid was the difference in fares. They did say we were eligible for the $300 euro per person payment for the flight delay. We flew in the same class and on the same day and we changed because they offered us a different flight. Should I appeal this or take the $300 euros? Thank you.