‘Change Role’ is WordPress built-in feature. It replaces current role of selected users for the selected role. If user does not have any role, selected role will be added to the user.
‘Grant Roles’, ‘Add Role’, ‘Revoke Role’ are added by User Role Editor.
‘Grant Roles’ makes the same as ‘Change Role’ but allows to replace all currently granted roles of selected users with selected roles. That is allows to work with multiple roles.
‘Add Role’ allows to add additional role to the roles currently granted to the selected users. For example, User1 has role A, after adding to him role B, he will have roles A and B.
‘Revoke Role’ allows revoke the role from the selected users. For example, User1 has roles A and B, after revoking to him role B, he will have role A only.