Move *Required Field Text to top

I would say that the easiest way to approach that would be to add a new required field markup somewhere in the beginning of the form. You could use wpmem_register_form_before (but that might be too early – before the heading), wpmem_register_form, or possibly slip some HTML in as part of the wpmem_register_form_args fieldset_before or main_div_before argument.

Then you could use wpmem_register_form_args to also remove the original markup.

So maybe something like this to just use the wpmem_register_form_args filter:

add_filter( 'wpmem_register_form_args', 'my_register_form_args' );
function my_register_form_args() {
	// required field HTML (change as desired)
	$req_field = '<div class="req-text">Required field<font class="req">*</font></div>';

	$args = array(
		//add req field html after the main_div_before tag
		'main_div_before'  => '<div id="wpmem_reg">' . $req_field,

		// empty the regular required field markup
		'req_mark'         => '',
		'req_label'        => '',
		'req_label_before' => '',
		'req_label_after'  => '',
	);
	return $args;
}