Sub SelectionReplaceTest()
    Dim rSelection  As Range    '// 選択セル範囲
    Dim r           As Range    '// １セル
    Dim sFind                   '// 検索文字列
    Dim sReplace                '// 置換文字列
    Dim f                       '// Formula値
    Dim v                       '// Value値
    
    sFind = "aaa"
    sReplace = "AAA"
    
    Set rSelection = Selection
    
    '// 選択セル範囲を１セルずつループ
    For Each r In rSelection
        f = r.Formula
        v = r.Value
        
        '// セルが数式の場合
        If (Left(f, 1) = "=") And (f <> v) Then
            '// 数式を置換
            r.Formula = Replace(f, sFind, sReplace)
        '// セルが数式でない場合
        Else
            '// Value値を置換
            r.Value = Replace(v, sFind, sReplace)
        End If
    Next
End Sub