They're trying to change it, because calling it Marine World implies to the GP that there's only... well, marine life/Sea World type stuff. But they also have a zoo portion and thrill rides. So they feel the name will better reflect the varity. Not 100% I agree, but I don't entirely disagree, either. Although it does seem a bit late to change the name now, after so many years with the old name.
Six Flags cannot close Marine World/Discovery Kingdom. The park is owned by the city and operations are handled by Six Flags. So at the most Six Flags could only stop their operations of the park and closure would have to be a city choice.