Changing Degree Programs, majors and levels
During the four year undergraduate degree program it is common for students to change their major or add additional major. The change can be triggered by change in interests or simply inability to handle a challenging major.
Changing to a different field of study
In any case of change of major two simple rules apply, your new major/department should be willing to take you and your old major / advisor is willing to release you, well, happily! Engineering programs have strict GPA requirements to advance to the next level. During the course of undergraduate program some students keep barely passing but fail to maintain the expected GPA. In such cases they have an option to switch to other majors with less stringent GPA requirements.
Changing degree levels and types within the same department
At grad school, the departments do not like for you to downgrade your degree level specially if you are funded. For example, if you are admitted for Ph.D. with funding and you want to bail out with an M.S. you may face problems. Similarly if you are a M.S. student with thesis option and you want to convert to a non-thesis option you are trying to move to an easier option and your department may not agree. The other way round is easier to do because that is considered a change from a easy option to a more difficult one.