A few points from a long ago PhD:
1. if a program has a joint MA/PhD program, they probably want to do all the grad training themselves. You might have difficulty going to another similar program later.
2. in the program i attended, if a student was not considered suitable to go on to a PhD, they were given a "terminal masters"--get your masters degree and get out. Leaving a PhD program after the masters would raise many questions for admissioins committees at other schools. Even some good recommendations might not resolve this issue.
3. the initial assignment to a professor is not set for all years. That depends on many factors which may change: grants received, teaching assistantships, faculty changes, death/retirement/disability of faculty..... Many students change interest areas after arriving. Over my grad career I had a number of funding sources not connected to my major professor.
Last suggestion...echoing others...if you got in and are getting money, consider yourself very fortunate and GO....
Old PhD
