Note: When using WordByWord, the first suggested change is automatically shown in the "Change To" text box, however when using AllAtOnce, it is not, but the "Change" button is enabled. Pressing the Change button then adds the suggestion to the "Change To" text box, but doesn't update the word above, so you have to press the Change button a second time.

Workaround:

public Form1()
{
    InitializeComponent();
    this.radSpellChecker1.SpellCheckMode = Telerik.WinControls.UI.SpellCheckMode.AllAtOnce;
    this.radSpellChecker1.SpellingFormShowing += radSpellChecker1_SpellingFormShowing;
}

private void radSpellChecker1_SpellingFormShowing(object sender, Telerik.WinControls.UI.SpellingFormShowingEventArgs e)
{
    RadButton changeButton = e.SpellingForm.Controls["buttonChange"] as RadButton;
    changeButton.PerformClick();
}