The emergence of SARS-CoV-2 underscores the need for strategies to rapidly develop neutralizing monoclonal antibodies that can function as prophylactic and therapeutic agents and to help guide vaccine design. Here, we demonstrate that engineering approaches can be used to refocus an existing neutralizing antibody to a related but resistant virus. Using a rapid affinity maturation strategy, we engineered CR3022, a SARS-CoV-1 neutralizing antibody, to bind SARS-CoV-2 receptor binding domain with >1000-fold improved affinity. The engineered CR3022 neutralized SARS-CoV-2 and provided prophylactic protection from viral challenge in a small animal model of SARS-CoV-2 infection. Deep sequencing throughout the engineering process paired with crystallographic analysis of an enhanced antibody elucidated the molecular mechanisms by which engineered CR3022 can accommodate sequence differences in the epitope between SARS-CoV-1 and SARS-CoV-2. The workflow described provides a blueprint for rapid broadening of neutralization of an antibody from one virus to closely related but resistant viruses.