Const adTypeText = 2
Const adSaveCreateOverWrite = 2

Dim inputFile, outputFile
    inputFile = "input_file.txt"
    outputFile = "output_file.txt"

Dim inputStream
    Set inputStream  = WScript.CreateObject("adodb.stream")
    With inputStream
        .Type = adTypeText
        .Charset = "unicode"
        .Open
        .LoadFromFile inputFile
    End With

Dim outputStream
    Set outputStream = WScript.CreateObject("adodb.stream")
    With outputStream
        .Type = adTypeText
        .Charset = "utf-8"
        .Open
        .WriteText inputStream.ReadText
        .SaveToFile outputFile, adSaveCreateOverWrite
    End With

    inputStream.Close
    outputStream.Close