A modification to a contract occurs when the parties agree to different terms than what they originally agreed to during formation. There are different rules for common law and the UCC that you must know. Generally, a modification to a contract can excuse non-performance. However, on an essay, you should address modification AFTER formation but BEFORE performance and breach. Why?
If you analyze the original terms, they will be breached, you will say the modification was a lawful excuse then you have to analyze everything again under the modified terms. Do you have time for that in exam world? Nope.
Rule tips: know common law, the modern trend with unanticipated circumstances in good faith and UCC 2-209.