Accepting a graduate offer of admission and then turning it down I was offered admission into a graduate physics program in February. I was given 21 days  within which to accept the offer and pay the admission deposit.

Now I was sent an email from the program offering me a prestigious university scholarship to motivate me to accept the offer of admission. I had not yet received any other offer of admission, so out of desperation, I accepted the offer of admission and paid the admission deposit.

Now, if I am offered admission into any of the upper grade universities I applied to, I will write to the graduate program director from the former program and politely mention that I cannot enroll in their program because the upper grade university is a better fit for me.

Will this cause a serious problem for me, i.e., in terms of my future academic career and the graduate program director's denial to let me go?
Traditionally, the recommendation is that as soon as you are accepted you withdraw your applications to schools that are out of the running. Then as soon as you accept an offer you withdraw your remaining applications. If a school make a decision early and requires a decision before you hear from schools you may find more desirable, it is best to ask for an extension.

I go against the norm a little and believe that if a school/employer will not give you and extension and wishes to put you at a disadvantage that it is not unreasonable to reneg on your acceptance. As this is not the norm, it will burn bridges.