What do I do if I really only figured out what I want to research just after applying
I saw this sort of thinking/discussion all the time. Something like, I'm a second year Ph.D. student but I want to start another one, because X is big and I'm excited about it. This is a bad way of thinking about a Ph.D. program. 

Here is the thing: 

One of the main pitfall of PhD students is that they can't get their mind into one problem.

Here is another fact: 

During Ph.D. you learn how to think and read/write as a researcher, regardless what you are working on.

Advice: I say choose one among all the offers you get. Drop all, unless you have a pretty good damn reason for it (e.g., got a job at NASA). 

Answering 'what I want to do for research' always changes! Thats the nature of research. You can not keep up with all trends during Ph.D. First, you need to think, read, and write as a researcher; and you learn them during a Ph.D. Then, you can follow the trends as a postdoc or an academic.