Yes, they must maintain licensed power and minimum hours of service. But they can apply to reduce the class of their license, which is routinely granted.
WBT could probably diplex somewhere with 50 kW days and 500 W nights. Maybe 1 kW nights. All non-directional. Hard to say precisely without the fancy software.
That's right. WBT is currently 50,000 watts non-directional by day. It only uses one tower in the daytime. So as PT Board Op says, it could ask the FCC to maintain 50 kW by day and reduce power at night to whatever avoids interference to KFAB Omaha, the other Class A station on 1110 AM.
It currently uses a three-tower array after sunset to protect KFAB while staying at 50 kW. So dropping the nighttime power while switching to a non-directional antenna would allow it to sell the current tower location. Then it could move to a single tower and share it with another AM station in the Charlotte market.