I need to vent.
Almost the same thing happened to us. We have a Chitin Dividend card that we have used and paid off for years (monthly). I think about 10 years. Anyway recently I paid the card off and took advance of a low interest offer from them.
Which gave me a running balance on the account. About two weeks ago we got two new cards in the mail with new numbers on them. Said that there had been some questionable acts put on the old account number so they issued us two new cards. I didn't do like you I did look at the cards and seeing they had different numbers I called them that is the answer they gave us. Not really true according to some talk on TV last night.
We went on and cut up the old cards Then we called and made the new number for the new cards