J'ai modifié comme suit :
Dim col, j As Integer
Dim c As Range
Set c = ActiveSheet.Rows(1).Find("Taux Princ.", LookIn:=xlValues)
If Not c Is Nothing Then
col = c.Column
Set c = Nothing
End If
For j = 1 To 10000
Cells(j, col) = Replace(Cells(j, col), ",", ".")
Next j
Merci de ta réponse !