In the worst case, the first university may contact with the second university, which then may think that you are an unqualified person and revoke your admission offer. But, this case seldom happens.
Most possibly, nothing will happen to you. But, your disobeying your previous words will cause some problems to the department. For example, your action will make another applicant loses his opptunity to be admitted, and your action will also make the department loses the funding assigned to you (if you are funded).
Peronally, I do think it is never a good idea to go back on your words.
Most possibly, nothing will happen to you. But, your disobeying your previous words will cause some problems to the department. For example, your action will make another applicant loses his opptunity to be admitted, and your action will also make the department loses the funding assigned to you (if you are funded).
Peronally, I do think it is never a good idea to go back on your words.