What if you formed a contract with a jazz band for its services?

Specifically, you wanted the band to play at your restaurant, because you thought that your customers would enjoy the band enough to pay to see it perform. You hired this specific band because you heard that it drew large crowds of paying customers. Imagine your surprise when, as you anticipate the band’s performance, you discover that another band—one you have never heard of—has come to play instead of the original contracting band. On inquiry, you learn that the original band transferred its duties to perform to a lesser known band. Can it do that? Under contract law, what permits the duties to be assigned?  What prevents the duties from being assigned? Is the original band still on the hook? Why or why not? How can you prevent this from happening in the future?