&CH1; is the entity placeholder and Bld1_Ch1.sgm is the file with the contents I want to copy in the master.

Here's the latest in my code. I'm having trouble getting the replace function to work on strMasterDoc.

Code:
Imports System
Imports System.IO
Imports System.Linq
Imports System.Text
Imports System.Windows.Forms
Imports System.Text.RegularExpressions


Public Class Form1
    Private Sub btnImport_Click(sender As Object, e As EventArgs) Handles btnImport.Click
        'Add a FolderBrowseDialog to your form designer
        FolderBrowserDialog1.ShowDialog()
        Dim searchDir As String = FolderBrowserDialog1.SelectedPath
        MsgBox("Search Dir " & searchDir)
        Dim existingFileMaster = Path.Combine(searchDir, "Bld1_Master_Document.sgm")
        MsgBox("fOUND MasterBuild file " & existingFileMaster)
        Dim lstFileChanges = CreateList(searchDir)

        'The following method does NOT return an array of lines
        Dim strMasterDoc = File.ReadAllText(existingFileMaster)
        For Each fc In lstFileChanges
            strMasterDoc = strMasterDoc.Replace(fc.OldString, fc.NewString)
        Next
        File.WriteAllText(existingFileMaster, strMasterDoc)
    End Sub

    Private Function CreateList(selectedPath As String) As List(Of FileChanges)
        Dim lstFC As New List(Of FileChanges)
        Dim count As Integer = Directory.GetFiles(selectedPath, "*.sgm").Count-1
        'MsgBox("COUNT  " & count)
        For i = 1 To count
            Dim fc As New FileChanges
            fc.OldString = $"&CH{i};"
            fc.FileName = $"Chapter{i}.sgm"
            fc.NewString = File.ReadAllText(Path.Combine(selectedPath, fc.FileName))
            lstFC.Add(fc)
        Next
        Return lstFC
    End Function
End Class

Public Class FileChanges
        Public Property OldString As String '&CH1.sgm 
        Public Property FileName As String 'Chapter1.sgm
        Public Property NewString As String 'Contents of Chapter1.sgm, the string to insert
    End Class