A fixed rate might be what sold you on the CapitalOne charge card -- but come September, the terms are changing for a lot of customers.

Notices are going out to some CapitalOne card holders that their fixed interest rate is changing to a variable rate that equals the prime rate plus 7.65 percent interest. That brings it to around 15.9 APR.

That would mean the difference between finance charges of $99 versus $159 on a $1,000 balance.

CapitalOne said the lending environment has changed since many of the accounts were opened and the changes take into account other factors like rising interest rates.