There is a contract between two parties for two years. Before the two years is up, they revise the contract and a new one is formed. If one party continues to follow the initial contract, can the other party sue?
The short answer to the question is "yes," a person can sue another for breach of contract if the person being sued as not complied with the terms of the second, or superceding, contract. Amending or replacing an existing contract is common practice, as the terms under which contracts are drafted and signed can change sufficiently as to make the existing contract illogical or irrelevant. Assuming that the new or amended contract replaces the provision of the original contract that is at issue, and assuming, logically, that both parties have consented to the revised contract, than it is legally binding.
The legal word for the situation described in the question is "novation," which means that the original parties obligations toward each other remain, but under new terms. If one party agrees to a revised or wholly new contract, and then fails to comply with the terms of the new agreement, he or she can be sued.