Are you saying that you changed all other cost codes successfully, except for the default? If so, can you temporarily replace the Default Cost Code setting with another already-updated cost code, then change the default cost code ID to match your new segmentation, then change the Default Cost Code again?
Default Cost Code can not be changed. In fact it is generated automatically by defined segmentation. Depending how you set your segments and their lengths, the platform will fill all segments by zero automatically.