I am a senior in high school and top 5% of my class in TX (graduating in May). I really should have applied to UT in September (like I was planning) in order to avoid this predicament, but alas. I decided on attending Texas A&M after visiting both UT and A&M campus and liking A&M’s better. I have realized now that I made a mistake because A&M offers a major of Allied Health and not nursing (in order to be an RN it would take a total of 6 years instead of 4). UT is perfect for my degree and even has the Master’s program I am interested in. What should I do? I can still apply for the spring semester at UT, but I would want to take college courses in the fall, as well. All advice is appreciated 
If you take classes in the fall, you’ll be considered a transfer admission. If you want to apply as a freshman you can’t be enrolled in another college.
Have you looked at the Texas A&M health sciences center bsn? It says 120 hours.
Texas A&M offers a 2 year BSN program in their health science center at Round Rock (not college station), this means you have to get your 2 years of prerequisite courses somewhere else. If you do your prerequisites at A&M in College Station, you can apply to the upper division in Round Rock as an internal transfer (I do not know if this would make you more competitive against normal transfer students).
If I remember correctly, UT no longer practices the 2 years of pre-reqs and 2 years of nursing school (this makes it harder for transfers because the majority of the graduating class is already decided, beginning freshman year rather than your junior year). I believe some important nursing classes begin during the 2nd semester of sophomore year so if you want to transfer to UT, you should do it as soon as possible.