C&D Services contracts with Ace Concessions, Inc., to service Aces vending machines. Later, C&D wants Dean Vending

Question:

C&D Services contracts with Ace Concessions, Inc., to service Ace’s vending machines. Later, C&D wants Dean Vending Services to assume the duties under a new contract. Ace consents. What type of agreement is this? Are Ace’s obligations discharged? Why or why not?