Change Application MainForm at runtime [duplicate]

Answer 1

No. Starting a second message loop on a single thread is not a valid operation. You can use Form.ShowDialog instead:

Hide();
MainForm newForm = new MainForm(newData);
newForm.Closed += (s, args) => Close();
newForm.ShowDialog();

Or if you'd like to close the old instance:

Hide();
MainForm newForm = new MainForm(newData);
newForm.ShowDialog();
Close();

Answer 2

"Is that possible"?

No.
Generally, you should run a sample and test. Then you'll answer the question yourself
You'll want to do something like:

private void button_click(object sender, RoutedEventArgs e)
{
    NextWindow window = new NextWindow();
    window.Show();
    this.Close();
}