Capital One owns Diners Club International, the company that operates the Diners transaction processing network and issues franchises to card issuers. The card issuing franchise for Canada and the USA is held by BMO, and is contracted to MasterCard network. Capital One would not be able to unilaterally change any of this, but it would be interesting to know when BMO's and MasterCard's contracts are up for renewal.
Citigroup sold Diners Club International to Discover Financial Services in 2008. The deal was announced in April 2008, completed on July 1, 2008, and included the international portion of the Diners Club business. Discover acquired the business for $165 million.
While the international business was sold to Discover, Citi also sold the North American franchise to Bank of Montreal (BMO) in 2009. This sale gave BMO the exclusive rights to issue Diners Club cards in the US and Canada.
I think Capital One needs to undo the damage to the brand that Citi did. Diners Club needs to become the Network for the Venture X and make it a competitor to Amex Platinum and then use Carte Blanche as a Centurion rival.