I have a similar problem. I will be studying communication and rhetoric at UPitt (*ahem*, in the PhD program, I omitted that information before this edit) in the fall, and I wrote in my SOP that I was most interested in focusing on the critical study of media and culture (in the critical theory, cultural studies, race, gender, class sort of way). The writing sample that I submitted also had to do with this type of thing. However, after this last semester of my undergrad (during which time I took a gender and media studies course that really turned me off to the whole deal, and not just in a superficial way), I have, for very good reason, become disenchanted with that entire research paradigm. I would like to focus instead on logic and rhetorical analysis, which I also have a strong background in from my undergrad (in fact, it is more substantial than my background in critical media studies), and which is also emphasized by the department at Pitt. However, I hesitate to tell my adviser (who is only my adviser temporarily, until I pick a permanent one) because I feel like it is inappropriate- especially given the fact that I was probably admitted based on the strong interest that I indicated in my SOP. In our correspondences, he has insinuated as much by making little side comments about which advisers (including himself) that I would probably want to work with full-time "given my interests", and has recommended that I take a certain course in that area of study, and so on.
So I feel like I will be somewhat of a disappointment when (and if) I tell him about this, which will have to happen, at the very latest, when I go to get my courses approved by him for registration and make a plan of study. I will have to tell him that I want to take "contemporary theories of rhetoric" instead of "media theory", and explain why I have changed my mind. Is this feeling warranted, do you think, given my specific situation? Or am I blowing this way out of proportion? In one sense, I think that it would be perfectly fine to switch my area of emphasis because it is still very much in line with the program, and the first year curriculum looks fairly lenient and exploratory. But I can't seem to get over the feeling that I have been deceptive, and that they might not have accepted me in the first place had my interests not been what they were in my SOP and writing sample.
I would like to apologize (and thank) in advance anyone who actually takes the time to read this rather long-winded sob story.