Bullnose with paper flanges
 

I am replacing square corners with bullnose and have the old corners pulled off. I think I need to scrape off more of the old coating to attach the paper to drywall, my husband says attach it to the existing wall. Which is the best way?

When drywall is installed for radiused corner bead--the drywall is set back from the corner 1/2" to allow the radiused bead to clear the drywall---so you will need to cut the drywall back so the corner is open---1/2" back from the face of the adjoining sheet(even with the face of the stud)

This could be done with a knife---or a Multi tool---