You are allowed to change your major when enrolled in AFROTC - several of my friends have done it - but it requires filling out forms and getting approval. Especially if you are trying to change from a "technical" major to a "non-technical" major. Ex., changing from aerospace engineering to political science. Changing from one technical major to another is typically easier since you're staying within the highly desired major realm.
Army and Marines typically don't care as much about your major. Navy option generally follows the same process as Air Force except Navy uses the tier system. But again, it's much easier to switch from a Tier 1 to a Tier 2 or vice versa than from a Tier 1 or 2 to a Tier 3 (non-tech) major.
Bottom line is you can't commission without getting your degree first so pick a major that you are interested in and can succeed in.