You are talking apples and cocunuts or I am bananas.
Or is used in a query as part of the WHERE condition.
Union is used to append queries together.

Give an example of your expected input data and then expected output.

OK, if anybody is interested in this I have put together a function that
works
for my particular situation where the OR applies to a particular field.
Not sure if Access is clever enough to automatically convert to the faster
method.

Function ConvertOr2Union(strSQL As String, _
                        Optional bUnionAll As Boolean = True) As String

 Dim i As Long
 Dim c As Long
 Dim x As Long
 Dim arrORS
 Dim arrORSWithRead
 Dim arrSQL
 Dim strUnion As String
 Dim strSQLUnion As String
 Dim lWHEREPos As Long
 Dim strAfterWHERE As String
 Dim lStart As Long
 Dim lEnd As Long
 Dim strReadPart As String
 Dim lUBReadBits As Long

 If bUnionAll Then
   strUnion = " UNION ALL "
 Else
   'this will remove duplicate rows and will be slower
   '--------------------------------------------------
   strUnion = " UNION "
 End If

 'get the OR parts after the WHERE
 '--------------------------------
 lWHEREPos = InStr(1, strSQL, "WHERE", vbTextCompare)
 strAfterWHERE = Mid$(strSQL, lWHEREPos + 5)
 arrORS = Split(strAfterWHERE, "OR", , vbTextCompare)

 'count the READ_CODE OR parts
 '----------------------------
 For i = 0 To UBound(arrORS)
   If InStr(1, UCase(arrORS(i)), "READ_CODE", vbBinaryCompare) > 0 Then
     c = c + 1
   End If
 Next i

 lUBReadBits = c - 1

 If lUBReadBits = 0 Then
   'no Read OR's so give original SQL back and get out
   '--------------------------------------------------
   ConvertOr2Union = strSQL
   Exit Function
 End If

 'set up an array with the READ_CODE OR parts
 '-------------------------------------------
 ReDim arrORSWithRead(0 To lUBReadBits) As String

 For i = 0 To lUBReadBits
   If InStr(1, UCase(arrORS(i)), "READ_CODE", vbBinaryCompare) > 0 Then
     arrORSWithRead(x) = arrORS(i)
     x = x + 1
   End If
 Next i

 'get the whole READ_CODE conditions string to be replaced
 '--------------------------------------------------------
 lStart = InStr(1, strSQL, arrORSWithRead(0), vbBinaryCompare)

 lEnd = InStr(InStr(1, strSQL, arrORSWithRead(lUBReadBits),
vbBinaryCompare), _
              strSQL, "AND", vbTextCompare)

 If lEnd = 0 Then
   lEnd = InStr(InStr(1, strSQL, arrORSWithRead(lUBReadBits),
vbBinaryCompare), _
                strSQL, "ORDER", vbTextCompare)
 End If

 If lEnd = 0 Then
   lEnd = Len(strSQL)
 End If

 strReadPart = Mid$(strSQL, lStart, lEnd - lStart)

 'now get the clean READ_CODE OR bits
 '-----------------------------------
 arrORSWithRead = Split(strReadPart, "OR", -1, vbTextCompare)

 'setup the new UNION SQL
 '-----------------------
 ReDim arrSQL(0 To lUBReadBits) As String

 For i = 0 To lUBReadBits

   If i = 0 Then
     'clear opening bracket
     '---------------------
     If InStr(1, arrORSWithRead(0), "(", vbBinaryCompare) > 0 Then
       arrORSWithRead(0) = Replace(arrORSWithRead(0), "(", "", 1, 1,
vbBinaryCompare)
     End If
   End If

   If i = lUBReadBits Then
     'clear closing bracket
     '---------------------
     If InStr(1, arrORSWithRead(lUBReadBits), ")", vbBinaryCompare) > 0
Then
       arrORSWithRead(lUBReadBits) = _
       Replace(arrORSWithRead(lUBReadBits), ")", "", 1, 1, vbBinaryCompare)
     End If
   End If

   arrSQL(i) = Replace(strSQL, strReadPart, arrORSWithRead(i), 1, 1,
vbBinaryCompare)

 Next i

 For i = 0 To lUBReadBits
   If i = 0 Then
     strSQLUnion = arrSQL(i)
   Else
     strSQLUnion = strSQLUnion & strUnion & arrSQL(i)
   End If
 Next i

 ConvertOr2Union = strSQLUnion

End Function

RBS