Bank of America Credit Card Complaint

Bank of America Credit card department,

APR or interest rate Ohio

I applied and received a Bank of America credit card 15-20 years ago. The card was issued with a 5.90 % fixed rate. I was just reviewing my statement and noticed that my rate has been changed to a 6.40 % variable rate. Is the issuing agency allowed to change my rate from a fixed ( as agreed ) to a higher variable rate without my knowledge or permission? I contacted Bank of America but they were unwilling to reverse the changes that they made.