This example checks to determine whether shape three on myDocument
is a callout with a custom drop. If it is, the code replaces the custom drop with one of two preset drops, depending on whether the custom drop value is greater than or less than half the height of the callout text box.
Set myDocument = ActivePresentation.Slides(1)
With myDocument.Shapes(3)
If .Type = msoCallout Then
With .Callout
If .DropType = msoCalloutDropCustom Then
If .Drop < .Parent.Height / 2 Then
.PresetDrop msoCalloutDropTop
Else
.PresetDrop msoCalloutDropBottom
End If
End If
End With
End If
End With