In template 'threadadmin_movethread' find:
<div><label for="rb_method_move"><input type="radio" name="method" value="move" id="rb_method_move" />$vbphrase[move_thread_to_destination_forum]</label></div>
<div><label for="rb_method_movered"><input type="radio" name="method" value="movered" id="rb_method_movered" checked="checked" />$vbphrase[move_thread_and_leave_redirect_in_previous_forum]</label></div>
And replace by:
<div><label for="rb_method_move"><input type="radio" name="method" value="move" id="rb_method_move" checked="checked" />$vbphrase[move_thread_to_destination_forum]</label></div>
<div><label for="rb_method_movered"><input type="radio" name="method" value="movered" id="rb_method_movered" />$vbphrase[move_thread_and_leave_redirect_in_previous_forum]</label></div>