Private Sub ButtonEdit_Click(sender As Object, e As EventArgs) Handles ButtonEdit.Click
    Dim Form2 As New frm2
    Form2.ComboBox1.text="test1_new"
    Form2.ComboBox2.text="test2_new"
...
    Form2.ShowDialog(Me)

 Private Sub ButtonEdit_Click(sender As Object, e As EventArgs) Handles ButtonEdit.Click
    Dim Form2 As New frm2
    инициализируйся, дура и больше не переопределяй сама то что ниже
    Form2.ComboBox1.text="test1_new"
    Form2.ComboBox2.text="test2_new"
...
    Form2.ShowDialog(Me)

 Private Sub ButtonEdit_Click(sender As Object, e As EventArgs) Handles ButtonEdit.Click
    Dim Form2 As New frm2
...
    Form2.ComboBox2.text="test2_new"
...
    Form2.ShowDialog(Me)

  Private Sub InitializeComponent()
Me.Form2.ComboBox2.text="test2"

 в ComboBox2_textchanged
Debug.Print (ComboBox2.text & " " ComboBox2.handle)"

'Конструктор:

Partial Class Form2
...
    Private Sub InitializeComponent()
    Me.ComboBox1 = New System.Windows.Forms.ComboBox()
    Me.SuspendLayout()
    '
    'ComboBox1
    '
    Me.ComboBox1.Items.AddRange(New Object() {"test1", "test2", "test3", "test4", "test5"})
    Me.ComboBox1.Location = New System.Drawing.Point(66, 46)
    Me.ComboBox1.Name = "ComboBox1"
    Me.ComboBox1.Size = New System.Drawing.Size(122, 21)
    Me.ComboBox1.TabIndex = 9
    Me.ComboBox1.Text = "test1"

'===========================

Public Class Form1

  Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim frm2 As New Form2
    frm2.ComboBox1.Text = "test1_new"
    frm2.ShowDialog()
  End Sub
End Class

Public Class Form1

  Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim frm2 As New Form2
    frm2.ComboBox1.Text = "test1_new"
    frm2.ShowDialog()
  End Sub
End Class

Public Class Form1
  Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim frm2 As New Form2
    'frm2.ComboBox1.Text = "test1_new"
    frm2.ShowDialog()
  End Sub
End Class

Public Class Form2
  Private Sub Form2_Load(sender As Object, e As EventArgs) Handles Me.Load
    Me.ComboBox1.Text = "test1_new"
  End Sub
End Class

Public Class Form1

  Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim frm2 As New Form2
    frm2.ComboBox1.Text = "test1_new"
    frm2.ShowDialog()
  End Sub
End Class