C'est bon, j'ai retrouvé la solution :
Sub Remplacer()
Dim f1 As Worksheet, f2 As Worksheet
Dim DerLig_A As Long, DerLig_B As Long, i As Long
Dim Valeur_X As String, Valeur_Y As String
Application.ScreenUpdating = False
Set f1 = Sheets("Feuillet_A")
Set f2 = Sheets("Feuillet_B")
DerLig_A = f1.Range("Z1").CurrentRegion.Rows.Count
DerLig_B = f2.Range("X1").CurrentRegion.Rows.Count
For i = 1 To DerLig_B
Valeur_X = f2.Cells(i, "X")
Valeur_Y = f2.Cells(i, "Y")
f1.Range("Z1:AA" & DerLig_A).Replace What:=Valeur_X, Replacement:=Valeur_Y, LookAt:=xlPart
Next i
Set f1 = Nothing
Set f2 = Nothing
End Sub
Merci à Arturo83