Originally Posted by ChrisSlicks

Do not drain the system! You can create air pockets inside the ABS module that are impossible to remove without the Nissan Consult computer.

When changing the calipers you should remove the old caliper and keep the line connected. Install the new caliper with new brake line and do the switch over as quickly as possible to avoid fluid loss. A thorough bleed of the system will remove 95% of the old fluid.