Changing from a nasal cannula to a mask is not that simple. The doctor has to order such and then he needs to prescribe the correct liter flow. The. Oxygen company has to make sure that your oxygen source is adequate for the mask and required liter flow. I would start with contacting the respiratory therapist at the oxygen company to help you through this.
I am a retired respiratory therapist who used to do home care so my info should be correct for you.
Have a blessed day