BP Amoco Corp., Chicago, will switch all employees over to a cash balance plan by next July, said Craig Weaver, manager of compensation and benefits for North America. Prior to the merger of BP America and Amoco, BP America had a $2.1 billion cash balance plan for its 4,000 employees. Amoco offered a traditional defined benefit plan with $2.8 billion in assets for its 18,000 employees. The manager structure for the plan has not yet been decided. Over the next 12 to 18 months, internal staff will review the investments and create the manager lineup for the new cash balance approach, according to a company spokesman.