The agreement you sign for ED specifically says that if you are a NEED BASED applicant that does not get enough aid to make it possible to attend, you are released from your agreement. I had the same worry when I applied, and looked into it. But it is only NEED BASED, not merit aid. Just make sure to check with the fin aid department that you qualify for this aid and that the stipulation applies to you. Also, admissions does not care about your financial situation (good or bad) when admitting, so that won’t affect you. However, they do not guarantee to meet full need if you are accepted. It depends on how badly they want to have you 
Here is an interesting link, if you are interested:
http://www.nytimes.com/2009/04/19/education/edlife/finaid-pappano-t.html