Русско-англо-русский юридический словарь

When things are going well and you want to continue the duration of an existing contract, a Contract Extension Agreement makes it easy. You don't have to create a whole new contract; with a Contract Extension Agreement, you can simply alter the end date of the original. (Rocket Lawyer)

An extension agreement is an agreement where the parties decide to change the end date of the original contract. (Contracts Counsel)