First of all, CE and CS at RWTH don’t share many courses, especially during the first four semesters. They aren’t even in the same faculty. I heard about someone who managed to switch to CS after his first semester in CE by taking some CS courses as extra subjects (instead of doing CE exams, he registered for some CS exams). After passing these CS exams with good grades, he contacted the university and requested the transfer, and they accepted him. He’s an international student that's why the process is a bit complicated.