Have StreamWriter write to the same location the StreamReader is on

I am writing a little app that opens up a logon script using a StreamReader and populates various controls such as text boxes and check boxes etc based on the values in the script.  I can get it to read properly and all is wonderful with it thus far.

The problem comes when I want to save the bloody thing.  I need to be able to write a value into the script in the same location that is in and I don't want to rewrite the ~ENTIRE~ script as well.  

I know one option would be to place the entire contents of the script into a vaiable, replace the text in question with new text then create a new file overwritting the old etc.  I don't want to do that if I can avoid it (Though I am exploring this path as well).

What I was doing is reading in the script again, line by line. When I get to a line that I know ~could~ be modified I check to see if it was changed through the UI and if it was, I want to overwrite the line with the new text.  Below is a sample of what I have thus far in code, but the text is getting written in the wrong location in my script, a sample of the script is further below.  What I was hoping I could somehow do is if I am reading a specific line, I want the writer to write directly to it, replacing the current text.  It seems that I should be using something other than a StreamReader and StreamWriter but I can't think of anything else.  I appreciate all the help in advance!

        Dim FS As FileStream = New FileStream(tScriptName.Text, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite)
        Using StreamReader As StreamReader = New StreamReader(FS)
            Dim sLine As String
            Do
                sLine = StreamReader.ReadLine
                If sLine.Contains("aSN                = Array(") = True Then
                    If bServerName = True Then      ' Name Change Only
                        Dim snTemp As String = "   aSN                = Array(" & String.Join(",", SNArray) & ")"
                        Using SW As StreamWriter = New StreamWriter(FS)
                            SW.WriteLine(snTemp)
                        End Using
                    End If
                ElseIf sLine.Contains("' End") Then
                    Beep()
                End If
            Loop Until sLine = "' End"
        End Using
        FS.Close()

Here is also a sample of the logon script:

   aOU                = Array("ou=torrance", "ou=irvine")
   aSN                = Array("\\CLS3", "\\irvinesvr\")      
   sDS               = "developer6"      ' Share Name

' Commands
   Call AdminPwd()      
   Call MapDrives()
   Call LogonScripts()
   Call CheckNAV()Start Free Trial