Switching from one area of graduate study to another? Is it possible to switch from one area of graduate study to another in US universities? For example, suppose someone has enrolled in a computer science phd program. Can he switch over to math(or physics) phd program in the same school later?(or say from applied mathematics to pure mathematics?)

What are the steps for doing for doing that?
In general, transferring to a different department requires a fresh application. While many departments will be sympathetic to applications from students alresdy at the university, they will still require all the formalities associated with a new admission. 

However, the procedures vary wildly from place to place, and you must check with your local institution (if you have one already). 

Other factors that can mitigate or complicate this process: if there's a standard procedure for doing this, if the two departments historically are comfortable with movement back and forth, if the departments are in the same college/school within the university, and so on.