I'm not sure I understand what you're asking... If you mean you are majoring in a certain thing, what you planned to get your degree in, and you've decided you'd rather major in & get your degree in something different... yes, you can switch majors. Depending on how far along you are in school, you might need to go to school longer, but if you'd much rather get your degree in something else, it's worth it to switch. :)
Whether you can stop your diploma course to join another course may depend on the level you are at. It is always possible to abandon one course for another when you are in the first year of your education. However, you may need to consult your dean of studies and find out if it would be possible for you to choose for another course at the level you are at.