French emails of the day: travelling by train on a whim, and updating your operating system

I continue to need to look up at least one word per French email. Goal: be able to read at least one email per day without having to look anything up…

• coup de tête: a sudden impulse.  When talking about soccer or violence, it’s a head-butt.  A couple of years ago I bought train tickets on line, and ever since have gotten emails with sentences like Vous souhaitez partir à petits prix sur un coup de tête?
  Related phrases:
  • donner un coup de tête à qqn: to head-butt.
  • sur un coup de tête: on an impulse, on a whim.
• dégressif: decreasing gradually.  The train company suggests that I Profitez d’un tarif dégressif en fonction du nombre de passagers voyageant ensemble (jusqu’à 6 passagers).
• passer en: not sure what this means.  Anyone?  Here’s the sentence, from a work email about upgrading operating systems:
  Si, au retour de vacances, votre ordinateur vous propose de passer en
  Ubuntu 14.04.1, mieux vaut refuser, en attendant le retour de l’expert
  Ubuntu (Olivier) le 8 septembre !