You could use a macro to loop through all the cells--or just the cells you're
using:
Option Explicit
Sub testme()
Dim myCell As Range
Dim myRng As Range
With ActiveSheet
Set myRng = .UsedRange
For Each myCell In myRng.Cells
If myCell.Style = "BadStyle" Then
myCell.Style = "GoodStyle"
End If
Next myCell
End With
End Sub
Replace badstyle and goodstyle with the styles you want.
If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm
Be aware that this code only looks at the used range--so if you applied a style
to a whole column, this macro would only do the change in the used range area
(between the top left cell of the worksheet (usually A1, but not always) and the
cell you get when you do ctrl-end).