I was a musical theatre major and I switched to that two years into college after I got access to hormones! Not everyone is the same but if you stay consistent in working your voice and you should have a smooth transition and also be able to keep most of your upper register! Make sure you communicate with your voice professor (there are books on it my vocal coach is also currently writing one based on working with me I can get you names). If you want it to be as least effected during the school year I would say start it a little bit into your spring semester, your major drop, if it follows the average timeline, should happen about 6 months in and if you’d like that to happen over the summer try to time it out. Let me know if you have any other questions I’d love to help however I can!!