Final answer:
After a contract has been signed, changes are known as addenda or amendments, with addenda adding terms without altering the original contract and amendments changing existing terms with all parties' agreement.
Step-by-step explanation:
Changes after a contract has been fully signed can be referred to as either addenda or amendments. The distinction between the two terms is that an addendum is typically used to include additional terms or clauses without altering the original agreement, whereas an amendment is used to make changes to the existing terms of the contract. Both require agreement from all parties involved in the contract and may include additional details such as changes in payment terms, interest rates, or procedures for change orders.