It can be changed in theory. If the completed research content is inconsistent with the existing topic, it should be changed, but the procedure is very troublesome and may not be successful. You should be able to download the application form for changing the topic on the website of the graduate school (department) of this school. Please have a look first. If there is no standard form, write your own application. Then the usual process is that the tutor signs-the department approves-and reports to the school.