Perhaps surprisingly, Moodle does not really have a concept of a course being finished, and the functionality you ask for does not exist.
As the moment, people who need this type of functionality normally integrate Moodle with some other system, for example a student information system. For instance you might export the grades from the Moodle gradebook to the other system, which then analyses the data, and then enrols students in another course.
Course completion, and prerequisites, is on the Roadmap for Moodle 2.0, so the functionality you want is coming inside Moodle.
Courses and course formats
Course prerequisites
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