In some situations, when working with the Graduated color or Graduated symbols symbology methods, when the number of breaks is changed on the Symbology tab of the Layer Properties dialog box, all the class breaks are lost. This only happens if the number of features in the data exceeds the maximum sample size.
If there are existing Graduated colors or Graduated symbols on a large number of features (larger than the current sample size) and the Symbology dialog box is reopened and the number of classes is changed, the classification is lost. This is because a rescan of the data is not triggered.
To change the number of classes:
1. Click 'Classify' to open the 'Classification' dialog box.
2. Change the number of classes in the 'Classes' drop-down list.