One thing to keep in mind is that Citi sold Discover the network, not the franchisees. Discover Network is strong in North America but weak overseas, and Diners Club is non-existent as a network in North America (cards issued in North America by Citi work on the MasterCard Network instead).

From various press releases and answers from Citi & Discover, it sounds like North American issued Diners Club cards will continue to be issued on the MasterCard Network. The overseas Diners Club Network and the Discover Network will be consolidated into one network over the next 2-3 years. They have not announced what the branding on the new combined network will be, but they did say that the Diners Club brand will not be going away, and that Discover network cards will be accepted anywhere Diners Club is accepted overseas, and Diners Club Network cards would be accepted anywhere Discover Network is accepted (presumably not including the MasterCard Network ones issued in the U.S. & Canada though).

Discover also said that they would not be issuing Diners Club cards themselves, leaving that to the franchisees, including Citi who is still a franchisee of Diner Club even after the Diners Club Network was sold to Discover. So the Discover name will not be going away either.

Although it is a bit unclear by press releases they have issued and answers they have provided so far, it appears that overseas Diners Club cards may switch from being accepted as MasterCard to being accepted as Discover Network when used in North America. Cards issued outside North America might even have the Discover Network logo on them, similar to how the MasterCard logo is on them now.

It should be interesting how this plays out.