find character and Insert a new line in the file - vb.net

Question

I am looking to read a file line by line and once I reach a line that doesn't contain the same characters in columns 65 - 67 I want to insert a string line 2 lines above that. I am using VB.NET
So far I have written code that reads the file line by line.
I am unsure how to insert a string line to a file where characters in columns 65-67 are different
any help would be great. Thanks
Protected Sub UploadFile(sender As Object, e As EventArgs)
Dim fileName As String = Path.GetFileName(FileUpload1.PostedFile.FileName)
Dim filePath As String = Server.MapPath("~/Files/") & fileName
FileUpload1.SaveAs(filePath)
'--Read block of lines then insert line for each new block
For Each line In IO.File.ReadLines(filePath)
'If line.Substring(65, 3) <> " " Then '<> to previous substring
'--Insert new line
'End If
Next
End Sub
<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="Button1" Text="Upload File" runat="server" OnClick="UploadFile" />

Answer 1

Something like this.
Dim lines As List(Of String) = IO.File.ReadLines(filePath).ToList
Dim idx As Integer = 0
Do While idx < lines.Count - 2
Dim thisLine As String = lines(idx)
Dim nextLine As String = lines(idx + 1)
If thisLine.Substring(65, 3) <> nextLine.Substring(65, 3) Then
lines.Insert(idx + 1, "NEW BLOCK")
idx += 1
End If
idx += 1
Loop
Dim foo As String = String.Join(ControlChars.Cr, lines)
IO.File.WriteAllText("path", foo)
edit - based on comments
Dim lines As List(Of String) = IO.File.ReadLines(filePath).ToList
Dim idx As Integer = 0
Do While idx < lines.Count - 2
Dim thisLine As String = lines(idx)
Dim nextLine As String = lines(idx + 1)
Dim key As String = thisLine.Substring(65, 3).Trim
If key <> nextLine.Substring(65, 3) AndAlso
key = "" Then
lines.Insert(idx + 1, "NEW BLOCK")
idx += 1
End If
idx += 1
Loop
Dim foo As String = String.Join(ControlChars.Cr, lines)
IO.File.WriteAllText("path", foo)