A recent study on the timing of declaring your college major by the Education Advisory Board (EAB) indicates that changing your major as late as your senior year does not necessarily conflict with your ability to graduate on time, contrary to popular beliefs. There is a nice article summarizing this on Inside Higher Ed for those who want the shorter version.
We certainly need to know more about the impact of college major. When is the best time to have students declare a major? Early on makes it more likely that the student will be able to complete all the requirements on time. But forcing that choice too early can mean making a decision that is not all that well informed. In the EAB study, students who declared early and stuck with that major were slightly less likely than the late major changers to graduate.
There are a lot of other things to consider here. First off, the study looked at when students officially declared their major, which might not be when they actually decided to switch and changed their course plan accordingly. It certainly could be the case that they just saved the paperwork until later in the game.
In looking at the impact of major change one also needs to take into account how drastic the change is. When I directed the Cooperative Institutional Research Program (CIRP) we looked at major changes and many of them were in the same related area: marine biology to biology, for instance. These are likely to share common courses that would could for either major, rather than, say, marine biology to art history. The latter change is likely to need additional courses.
Then to throw another wrench into the picture, there are double majors, also a topic of recent examination. In another new study, students with double majors scored higher on a measure of innovation than those with only one major. The authors then tied this into research showing that innovation is a desirable trait that employers look for.
The impact of major is key to the college experience, and it's great that we are learning more about how to help students navigate this important decision.