I was a biology major in undergrad but took several sociology courses as well. I have been successful in applying. Just take a good amount of time in your personal statement explaining the reason for your change in interests, what motivated it, what you're interested in now, and why that fits into their specific program. I also had 1.5 years work experience post-undergraduate, and was able to talk about how the parts of that job I enjoy would support sociological study well.