I was accepted and I selected poster, but the conference organizers are telling me I have to give a talk or I was accepted and I selected talk, but they are telling me I have to give a poster.

Talk and poster selection will be done by the conference committee, and it depends on availability. If you selected poster but we notify you that you can only give a talk, you have the option to accept or reject that, and vice versa. If you reject, then we will not be able to accommodate your research.