The following VBA example moves all flat drop shadows - so that the offset distance is 0.5" - while preserving the original offset angle.
Sub Test()
Const Radius As Double = 0.5
Dim s As Shape, ds As EffectDropShadow
Dim dx As Double, dy As Double, r As Double
For Each s In ActivePage.Shapes
If s.Type = cdrDropShadowGroupShape Then
Set ds = s.Effect.DropShadow
If ds.Type = cdrDropShadowFlat Then
dx = ds.OffsetX
dy = ds.OffsetY
r = Sqr(dx * dx + dy * dy)
If r = 0 Then
dx = 1
r = 1
End If
ds.OffsetX = dx * Radius / r
ds.OffsetY = dy * Radius / r
End If
End If
Next s
End Sub