xl2002+ gives you the ability to do Edit|Replace based on format.
If you have mixtures of other fonts that shouldn't be changed, then I think
you'll have to do it manually or have a macro that loops through each cell
changing the font.
Kind of like:
Option Explicit
Sub testme()
Dim wks As Worksheet
Dim myCell As Range
Set wks = ActiveSheet
For Each myCell In wks.UsedRange.Cells
If myCell.Font.Name = "Gunsuh" Then
myCell.Font.Name = "Arial"
End If
Next myCell
End Sub
(I don't have Gunsuh on my pc. Make sure you spell it correctly.)
If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm