You need to tell Word to match the Find/Replace case:
Code:
With Selection.Find
.ClearFormatting
.Replacement.ClearFormatting
.Forward = True
.MatchCase = True
.Wrap = wdFindContinue
.Text = "Á"
.Replacement.Text = "A"
.Execute Replace:=wdReplaceAll
.Text = "á"
.Replacement.Text = "a"
.Execute Replace:=wdReplaceAll
End With