When a character loses "all keywords," it loses one instance of each keyword. For "qualified keywords" such as Prized and Immunity, they will lose once instance of each qualification. So, if the character with printed "Prized 2" and gained a "Prized 1" was facing Men of Pride, it would lose one instance each of "Prized 1" and "Prized 2" (for a grand total of no effective keywords).
If this is hard to picture, imagine that I have Warlock Servitors out (immune to character abilities). I reveal the plot Calm Before the Storm (characters and locations are immune to events). My Warlock Servitors are "Immune to character abilities. Immune to events." (Not "Immune to character abilities and events." See the subtle difference?) But you revealed Men of Pride. So what immunity does my Warlock Servitors lose? Well, when they lose "all keywords," they lose an instance of "Immune to events" and a separate instance of "Immune to character abilities" - leaving them with no effective immunity.
That's kind of a long way of saying Each distinct qualification of a particular keyword (like immunity, "No attachments" or Prized) is considered a different keyword for determining the outcome when a card gains or loses "all" keywords.