It's difficult for us to know exactly what Citi did because they were both the franchise issuer and the franchise holder in many markets. Diners Club International is the name of the entity that operates the Diners transaction network -and- grants franchises to the various card issuers, including to BMO for Canada and the USA. Citi migrated the Canadian and USA franchises to MasterCard network while they were still the franchise holder in those markets.

This suggests that there is a franchise agreement with BMO -and- also a contract with MasterCard. It was evident early on that BMO was primarily interested in the Corporate card issuing business, and I do not imagine that they would be motivated to migrate those accounts from MasterCard to Diners/Discover. However, they might be willing to sell the personal/small business franchise.