fix #4331

Author: jef-n

src/app/qgsuniquevaluedialog.cpp

@@ -285,7 +285,7 @@ void QgsUniqueValueDialog::changeClassificationAttribute()
                               tr( "The classification field was changed from '%1' to '%2'.\n"
                                   "Should the existing classes be deleted before classification?" )
                               .arg( mOldClassificationAttribute ).arg( attributeName ),
-                              QMessageBox::Ok | QMessageBox::Cancel ) == QMessageBox::Ok )
+                              QMessageBox::Yes | QMessageBox::No ) == QMessageBox::Yes )
   {
     mClassListWidget->clearSelection();
     deleteSelectedClasses();