A supplement to a contract is used to update or modify an existing contract, which is often the case for business relationships. Cancelling and creating a new contract is not ideal considering the cost and time it would take to make this change. Instead, it`s much easier to maintain the existing contract and use an endorsement to make subtle, even major changes, as you think. Note that the addendum in style, font, and language should conform to the original agreement. A supplement to a contract should also be signed by the same signatories as the original treaty and, where appropriate, by additional signatories. Homesteps file number: Addendum # 1 to the sales contract (transfer of individual ownership) This addendum is part of the agreement (sales contract) of the 20th.