Am I stuck with the topic/advisor that recommended my admission?
This is ultimately dependent on the policies of your department. The basic issue would be that if you switch to another advisor, the other advisor would presumably be responsible for covering the remainder of your stipend. The big question is if you are free to choose both advisor and topic, or if it's expected you'd stick with your advisor, but have freedom to choose the PhD topic.

The right point of contact is the graduate advisor, who will be well versed in the procedures of your specific department.