Since CI moved out, the government has decided to let HND be used for international 'charter' services. They are limited in numbers. The transport minister a few months ago came up with the idea of allowing for the use of HND for 'emergency' situations when NRT is closed. NRT has a strict curfew from 11 p.m. until 6 a.m., I believe.
I really wish the government didn't build NRT from the beginning. No body seems to know now how and who exactly came up with the idea of NRT airport.