Consequence of declining an accepted offer after the universal deadline 15th April This is my cousin's problem.  In the first week of March he was offered a PhD offer (Upper Middle ranked School in USA) in Physics and he  received his I-20 to get the visa.The admit was fully funded. He accepted the offer because he didn't have any offer at that time.  But few days ago, a top ranked school admin said to him that, he might be admitted as a lot of students haven't accepted their offer (it is not guaranteed though plus admission is completed ). 

He was always expecting to get admitted in that school. The deadline of the offer letter from first school  is 15th April (He accepted the offer already).

Will it be possible to decline the first offer later/admission  if he gets the offer after around two weeks from the top ranked school? I want to know the consequences of this situation. 

EDIT:

I have received another suggestion from academia. He suggested to defer the admission (if he gets from the second university) from the Top ranked school. After a semester completed in the first admitted University, If my cousin feels that he should change the department, he will be able to do it. How reasonable is this?
Assuming both schools are members of the Council of Graduate Schools, they have an agreement about cases like this. If you don't change your mind and reject the offer from the first school by April 15, you have to get a letter from the first school releasing you from your obligation to attend before accepting the second school. Otherwise, the second school will be unable to take you.

In the one case I know about, there wasn't any problem in obtaining this letter. However, I can't promise that all graduate schools will be as reasonable. 

If your cousin gets an offer from the second school before April 15, he should contact the admissions officers at the first school by April 15 (today!) and tell them he is rescinding his acceptance and why.

If he gets an offer after April 15, he needs to make sure the first school is okay with him not coming and get a letter from them before he accepts the second school.