Unless you’ve applied to liberal arts colleges where you’re given at least a year before having to decide on a major, you have to apply and audition before being admitted as a music major. And yes, it would be too late for the upcoming Fall semester. Also, your audition needs to be strong enough to gain admission. That said, if you continue to take lessons and practice a lot, you may be able to consider changing your major by auditioning for the following year. If you do this, be sure to find out if you can take introductory music theory in your freshman year so you have a chance to graduate in four years. Another option would be to try to pick up a music minor or at least continue taking lessons with faculty or upper level students or grad students, and audition for ensembles, a cappella groups, and other music groups on campus.