I don’t think that plan makes sense. You say you don’t have enough training to get into a musical theater program. But if you get into an acting program, all your training would be in acting, NOT musical theater…so how would that help you transfer to the musical theater major? In fact, you would be even MORE behind, because the musical theater majors would have had a full freshman year of musical theater training, while your freshman year would be all acting.
I think your best bet is to take private singing lessons NOW (and dance if you can fit some in, but definitely singing). Even a few months of singing lessons can make a big difference. Definitely start finding monologues NOW and working with an acting coach, as well as working on songs. Then apply to schools that have both acting and musical theater programs, and apply to BOTH of those programs within each school. That way, you have a chance at getting in for musical theater OR acting.
Another option is to go for a non-audition BA program which has a theater program. That way you would only have to worry about getting accepted academically, without worrying about auditions. Here is a full list of colleges that offer MT, including the category of non-audition BA programs: http://talk.collegeconfidential.com/musical-theater-major/1335445-big-list-of-mt-colleges-by-program-type-p1.html
Also, for musical theater questions in general, definitely read the musical theater forum here on CC: http://talk.collegeconfidential.com/musical-theater-major/
Good luck!