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