Final answer:
The only way to downgrade a confined space is to remove the hazardous materials, which mitigates the dangers and may change the classification of the space.
Step-by-step explanation:
The only way to downgrade a confined space referenced in the question would be to remove the hazardous materials. Downgrading a confined space means to eliminate or control hazards to a point where the space no longer meets the definition of a confined space or is considered a permit-required confined space. This can include removal of contaminants or improvement of atmospheric conditions. Increasing ventilation can improve the air quality, but it does not necessarily downgrade the space. Decreasing the oxygen level could make the space more hazardous. Thus, the correct answer from the options provided is to remove the hazardous materials.