This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
If the new contract abrogates the earlier contract by express terms, no question of the intention of the parties can usually arise.1 In such case it abrogates it in toto unless some restriction is made in the later contract, preventing such total abrogation.2
In order to operate as a discharge or modification of an earlier contract, the new contract must appear to have been so intended. A new contract of equal degree with an earlier one, and upon the same subject-matter, will not abrogate or merge the earlier contract, if the parties make an express agreement that it shall not so operate.3 A new contract intended to confirm and ratify a prior contract can not operate as a discharge thereof.4