If (cb.SelectedIndex = 1) Then
                'MessageBox.Show("Selecciono {0}", "Mensaje", MessageBoxButtons.OK, MessageBoxIcon.Error)
                'cmbemp.Focus()
                Dim comparisonType As StringComparison = StringComparison.OrdinalIgnoreCase
                FindFileAndReplaceTextIn(sourceDir, "GL.txt", "MCBO-02", "MBO-03", comparisonType)
                FindFileAndReplaceTextIn(sourceDir, "GL.txt", "MCBO-03", "MBO-03", comparisonType)
                FindFileAndReplaceTextIn(sourceDir, "IMP6000.txt", "TXT01", "MBO-03", comparisonType)
                FindFileAndReplaceTextIn(sourceDir, "IMP6000.txt", "TXT02", "MBO-03", comparisonType)
            Else
                Exit Sub
            End If

        Public Sub FindFileAndReplaceTextIn(...)
         ...
             Dim txt As String = File.ReadAllText(txtfile.FullName, Encoding.Default).
                                      Replace(txtfind, txtreplace, comparisonType, 128)
         ...
        End Sub

        Public Function Replace(...) As String
 
                Return StringExtensions.Replace(sender, findWhat, replaceWith, comparisonType, stringBuilderCapacity:=0)
 
        End Function

        Public Sub FindFileAndReplaceTextIn(...)
         ...
             Dim txt As String = File.ReadAllText(txtfile.FullName, Encoding.Default).
                                      Replace(txtfind, txtreplace, comparisonType, 128)
         ...
        End Sub

        Public Sub FindFileAndReplaceTextIn(...)
         ...
             Dim txt As String = File.ReadAllText(txtfile.FullName, Encoding.Default).
                                      Replace(txtfind, txtreplace, comparisonType)
         ...
        End Sub

' ESTE MODULO ESTÁ INCOMPLETO, SOLO LE AÑADÍ LAS FUNCIOENS DE REEMPLAZAMIENTO DE TEXTO.
' SI QUIERES EL CÓDIGO FUENTE COMPLETO CON MÁS FUNCIONES INTERESANTES, PUEDES DESCARGARLO AQUÍ:
' https://github.com/ElektroStudios/VBNetSnippets/blob/master/String/String%20Extensions.vb
 
#Region " Public Members Summary "
 
#Region " Functions "
 
' String.Replace(String, String, StringComparison) As String
' String.ReplaceRegEx(String, String, RegExOptions) As String
 
#End Region
 
#End Region
 
#Region " Option Statements "
 
Option Strict On
Option Explicit On
Option Infer Off
 
#End Region
 
#Region " Imports "
 
Imports System
Imports System.Linq
Imports System.Diagnostics
Imports System.Runtime.CompilerServices
Imports System.Text
Imports System.Text.RegularExpressions
 
#End Region
 
Namespace Tools
 
    ''' ----------------------------------------------------------------------------------------------------
    ''' <summary>
    ''' Contains custom extension methods to use with a <see cref="String"/>.
    ''' </summary>
    ''' ----------------------------------------------------------------------------------------------------
    Public Module StringExtensions
 
#Region " Public Extension Methods "
 
        ''' ----------------------------------------------------------------------------------------------------
        ''' <summary>
        ''' Replaces text using a regular expression.
        ''' </summary>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <example> This is a code example.
        ''' <code>
        ''' Dim str As String = "Hello World!".ReplaceRegEx("world", "kitty", RegexOptions.IgnoreCase)
        ''' </code>
        ''' </example>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <param name="sender">
        ''' The source <see cref="String"/>.
        ''' </param>
        ''' 
        ''' <param name="findWhat">
        ''' The <see cref="Regex"/> find expression.
        ''' </param>
        ''' 
        ''' <param name="regexOptions">
        ''' The <see cref="RegexOptions"/>.
        ''' </param>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <exception cref="ArgumentNullException">
        ''' findWhat
        ''' </exception>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <returns>
        ''' The replaced string.
        ''' </returns>
        ''' ----------------------------------------------------------------------------------------------------
        <DebuggerStepThrough>
        <Extension>
        Public Function ReplaceRegEx(ByVal sender As String,
                                     ByVal findWhat As String,
                                     ByVal replaceWith As String,
                                     Optional ByVal regexOptions As RegexOptions =
                                                    RegularExpressions.RegexOptions.None) As String
 
            If String.IsNullOrEmpty(sender) Then
                Return sender
 
            ElseIf String.IsNullOrEmpty(findWhat) Then
                Throw New ArgumentNullException(paramName:="findWhat")
 
            Else
                Return Regex.Replace(sender, findWhat, replaceWith, regexOptions)
 
            End If
 
        End Function
 
        ''' ----------------------------------------------------------------------------------------------------
        ''' <summary>
        ''' Replaces text using the specified string comparison type.
        ''' </summary>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <example> This is a code example.
        ''' <code>
        ''' Dim str As String = "Hello World!".Replace("world", "kitty", StringComparison.OrdinalIgnoreCase)
        ''' </code>
        ''' </example>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <param name="sender">
        ''' The source <see cref="String"/>.
        ''' </param>
        ''' 
        ''' <param name="findWhat">
        ''' The <see cref="Regex"/> find expression.
        ''' </param>
        ''' 
        ''' <param name="comparisonType">
        ''' The string comparison type.
        ''' </param>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <exception cref="ArgumentNullException">
        ''' findWhat
        ''' </exception>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <returns>
        ''' The replaced string.
        ''' </returns>
        ''' ----------------------------------------------------------------------------------------------------
        <DebuggerStepThrough>
        <Extension>
        Public Function Replace(ByVal sender As String,
                                ByVal findWhat As String,
                                ByVal replaceWith As String,
                                ByVal comparisonType As StringComparison) As String
 
            If String.IsNullOrEmpty(sender) Then
                Return sender
 
            ElseIf String.IsNullOrEmpty(findWhat) Then
                Throw New ArgumentNullException(paramName:="findWhat")
 
            Else
                Return StringExtensions.InternalReplace(sender, findWhat, replaceWith, comparisonType, stringBuilderCapacity:=-1)
 
            End If
 
        End Function
 
#End Region
 
#Region " Private Methods "
 
        ''' ----------------------------------------------------------------------------------------------------
        ''' <summary>
        ''' Replaces text using the specified string comparison type.
        ''' </summary>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <example> This is a code example.
        ''' <code>
        ''' Dim str As String = "Hello World!".Replace("world", "kitty", StringComparison.OrdinalIgnoreCase)
        ''' </code>
        ''' </example>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <param name="sender">
        ''' The source <see cref="String"/>.
        ''' </param>
        ''' 
        ''' <param name="findWhat">
        ''' The <see cref="Regex"/> find expression.
        ''' </param>
        ''' 
        ''' <param name="comparisonType">
        ''' The string comparison type.
        ''' </param>
        ''' 
        ''' <param name="stringBuilderCapacity">
        ''' The initial buffer size of the <see cref="Stringbuilder"/>.
        ''' This parameter is reserved for testing purposes.
        ''' </param>
        ''' ----------------------------------------------------------------------------------------------------
        ''' <returns>
        ''' The replaced string.
        ''' </returns>
        ''' ----------------------------------------------------------------------------------------------------
        <DebuggerHidden>
        <DebuggerStepThrough>
        Private Function InternalReplace(ByVal sender As String,
                                         ByVal findWhat As String,
                                         ByVal replaceWith As String,
                                         ByVal comparisonType As StringComparison,
                                         ByVal stringBuilderCapacity As Integer) As String
 
            Dim posCurrent As Integer = 0
            Dim lenPattern As Integer = findWhat.Length
            Dim idxNext As Integer = sender.IndexOf(findWhat, comparisonType)
            Dim result As New StringBuilder(capacity:=If(stringBuilderCapacity <= 0, Math.Min(4096, sender.Length), stringBuilderCapacity))
 
            While (idxNext >= 0)
                result.Append(sender, posCurrent, (idxNext - posCurrent))
                result.Append(replaceWith)
 
                posCurrent = (idxNext + lenPattern)
                idxNext = sender.IndexOf(findWhat, posCurrent, comparisonType)
            End While
 
            result.Append(sender, posCurrent, (sender.Length - posCurrent))
 
            Return result.ToString
 
        End Function
 
#End Region
 
    End Module
 
End Namespace

Using sr As StreamReader = txtfile.OpenText
 
    Using sw As New StreamWriter(Path.Combine(topDir.FullName, curFilename), append:=True, encoding:=Encoding.Default, bufferSize:=4096)
 
        Dim content As String = sr.ReadToEnd.Replace(Environment.NewLine, "")
        If Not String.IsNullOrEmpty(content) Then
            sw.WriteLine(content)
        End If
 
    End Using ' sw
 
End Using ' sr

sw.WriteLine(content & Environment.NewLine)