I would like to modify the WP side menu

So I'm trying to modify the markup of the WP Admin side menu (#adminmenu). I am looking for a filter to add/remove classes from the UL and it's child LI's as obviously I don't want to edit core wp files directly in case of updates.

The reason for this is to remove the standard wp-admin side menu fly-out and replace with an accordion menu (using metisMenu)

I have found these resources.

But I am not finding a solution that allows me to edit markup with directly modifying wp-admin/menu-header.php.