Mileage associated with different individuals may not be combined. For example, Mr Chen has 40,000 miles and Mrs Chen has 20,000 miles. Mrs Chen is travelling from Taipei to Los Angeles, and wants to upgrade from Premium Economy Class to Business Class (Class K) at a cost of 31,500 miles.
This could be possible if Mr Chen transfers the full value of the upgrade (31,500 of his miles) to Mrs Chen. He cannot just transfer 11,500 miles to Mrs Chen to bring her total miles up to 31,500 and secure the upgrade.