I create (or better try) to produce my first WordPress theme with Less.

Things I do is by using a script like this into my functions.php

wp_register_style('screen_css', get_bloginfo('template_url') . '/css/screen.less', array(), false, 'screen');

and as a result:

<link rel='stylesheet' id='stigma_screen-css'  href='http://www.stigmahost.dch/wp-content/themes/stigmahost/css/screen.less?ver=1.0' type='text/css' media='screen' />

Now you ask ,, can one change in some way the rel="stylesheet" after i while using wordpress_register_style() function ?

While neither function enables you to pass that value in, you need to do have the tag prior to it being made using the style_loader_tag filter. Should you choose something similar to this...

add_filter('style_loader_tag', 'my_style_loader_tag_function');

function my_style_loader_tag_function($tag){
  //do stuff here to find and replace the rel attribute

  return $tag;

...you need to have the ability to replace the rel attribute with anything you want. Bear in mind this filter will pass within the whole tag as html, so you will need to perform a preg_replace() or something like that much like replace the worthiness with what you would like. Also, the hepa filter will run any time you enqueue a stylesheet, so make certain you test that you have the correct one (having a preg_match() or something like that) before you decide to affect the rel attribute.