Ed and Nathan are the only people authorized to replace ink or paper in the plotters. Do you really want to be responsible for breaking a $5000 machine?
We do not replace the ink cartridges until the plotter pauses a job (they start warning at ~20% life) as they are fairly expensive. The plotter has a nice feature where it won't ruin a print job just because the ink runs out; at ~1% it simply pauses the job, allows us to replace the empty cartridge, then resumes it. We haven't seen any faded print jobs and it is hard/impossible to tell where the cartridge is replaced.
Weekend and after hours (5:00 PM - 8:00 AM) plotting is not officially supported but is allowed if you know what you are doing. However we are not available for replacing supplies so we strongly recommend you don't wait until the last minute to print! As mentioned above we don't pre-replace the ink cartridges as there is still ~$50 worth of ink when it first starts the warning.