After analog signals are shut off (or, in fact, if a station chooses to shut off their analog early) then stations can still negotiate with cable, or they can force cable systems to carry only one (1) of their digital channels.
Ion Media, TBN, and others would vastly prefer "digital multi-cast must-carry" - a policy that would require cable systems to eventually carry as many channels as a station can transmit over their digital carrier. In most cases this will initially be four to six channels, but could eventually be as many as eight. However, only 3 Mhz of cable capacity is required to transmit an entire digital broadcast station, whether that station sends one HD channel or six SD channels. In comparision, analog must-carry of one channel uses 6 Mhz of capacity.