The boundary between the current geographic region served by the 323 and 213 area codes will be eliminated so that both area codes will serve the same geographic region. In other words, the two areas served by the 213 and 323 area codes are being combined and telephone numbers for either area code will be assigned in the new combined area. This is known as a boundary elimination area code overlay.
Note: A boundary elimination area code overlay doesn't require customers to change their existing area code but does require customers to dial at minimum the area code + telephone number on every call, including calls within the same area code.
For more information, read the customer letter below: