WP_User::set_role( string $role )

	public function set_role( $role ) {
		if ( 1 == count( $this->roles ) && $role == current( $this->roles ) )
			return;

		foreach ( (array) $this->roles as $oldrole )
			unset( $this->caps[$oldrole] );

		$old_roles = $this->roles;
		if ( !empty( $role ) ) {
			$this->caps[$role] = true;
			$this->roles = array( $role => true );
		} else {
			$this->roles = false;
		}
		update_user_meta( $this->ID, $this->cap_key, $this->caps );
		$this->get_role_caps();
		$this->update_user_level_from_caps();

		/**
		 * Fires after the user's role has changed.
		 *
		 * @since WP-2.9.0
		 * @since WP-3.6.0 Added $old_roles to include an array of the user's previous roles.
		 *
		 * @param int    $user_id   The user ID.
		 * @param string $role      The new role.
		 * @param array  $old_roles An array of the user's previous roles.
		 */
		do_action( 'set_user_role', $this->ID, $role, $old_roles );
	}