Here is some explanation about the constants
Word 2003 VBA Language Reference: Word Enumerated Constants
You can reuse the macro you created. Together with some imagination and
AdminScriptEditor that reveals all the arguments, you could write something like:
;*************************************************************************
; Script Name:
; Author: green78
; Date: 21/07/2007
; Description:
;************************************************************************* 

;Script Options
If Not @LOGONMODE
    Break On
Else
    Break Off
EndIf
Dim $RC
$RC = SetOption("Explicit", "On")
$RC = SetOption("NoMacrosInStrings", "On")
$RC = SetOption("NoVarsInStrings", "On")
If @SCRIPTEXE = "KIX32.EXE"
    $RC = SetOption("WrapAtEOL", "On")
EndIf

;Declare vaiables
Dim $strSrcFile, $strDstFile

Dim $wdOpenFormatAllWord, $wdOpenFormatAuto, $wdOpenFormatDocument, $wdOpenFormatEncodedText
Dim $wdOpenFormatRTF, $wdOpenFormatTemplate, $wdOpenFormatText, $wdOpenFormatUnicodeText
Dim $wdOpenFormatWebPages

Dim $wdFormatDocument, $wdFormatDOSText, $wdFormatDOSTextLineBreaks, $wdFormatEncodedText
Dim $wdFormatFilteredHTML, $wdFormatHTML, $wdFormatRTF, $wdFormatTemplate, $wdFormatText
Dim $wdFormatTextLineBreaks, $wdFormatUnicodeText, $wdFormatWebArchive, $wdFormatXML

Dim $wdCRLF, $wdCROnly, $wdLFCR, $wdLFOnly, $wdLSPS

Dim $objWord

;Initialize variables
$strSrcFile = "C:\test\test-UDF-8.txt"
$strDstFile = "C:\test\test-ANSI.txt"

$wdOpenFormatAllWord = 6
$wdOpenFormatAuto = 0
$wdOpenFormatDocument = 1
$wdOpenFormatEncodedText = 5
$wdOpenFormatRTF = 3
$wdOpenFormatTemplate = 2
$wdOpenFormatText = 4
$wdOpenFormatUnicodeText = 5
$wdOpenFormatWebPages = 7

$wdFormatDocument = 0
$wdFormatDOSText = 4
$wdFormatDOSTextLineBreaks = 5
$wdFormatEncodedText = 7
$wdFormatFilteredHTML = 10
$wdFormatHTML = 8
$wdFormatRTF = 6
$wdFormatTemplate = 1
$wdFormatText = 2
$wdFormatTextLineBreaks = 3
$wdFormatUnicodeText = 7
$wdFormatWebArchive = 9
$wdFormatXML = 11

$wdCRLF = 0
$wdCROnly = 1
$wdLFCR = 3
$wdLFOnly = 2
$wdLSPS = 4


$objWord = CreateObject("Word.Application")
If @ERROR
    ? "Error creating Excel object"
    ? "Error " + @ERROR + ": " + @SERROR
    Exit @ERROR
EndIf

;$objWord.Visible = True ;No need to make this visible
;$RC = $objWord.Documents.Open($strSrcFile,0,0,0,"","",0,"","",$wdOpenFormatAuto,65001,,,,,"")
$RC = $objWord.Documents.Open($strSrcFile) ;You will see this is enough
;$RC = $objWord.ActiveDocument.SaveAs($strDstFile,$wdFormatText,0,"",1,"",0,0,0,0,0,1252,0,0,$wdCRLF)
$RC = $objWord.ActiveDocument.SaveAs($strDstFile,$wdFormatText) ;You will see this is enough
$objWord.Application.Quit

Here is the essential and simplified part
Break ON
$strSrcFile = "C:\test\test-UDF-8.txt"
$strDstFile = "C:\test\test-ANSI.txt"
$objWord = CreateObject("Word.Application")
$RC = $objWord.Documents.Open($strSrcFile)
$RC = $objWord.ActiveDocument.SaveAs($strDstFile,2)
$objWord.Application.Quit