words

There must be a term for words/expressions that are coming/have come to imply their opposite. such as

• sanitary
  
• "I'll be right with you"
  
• "Your call is important to us"
  
• "Thank you for sharing"
  
• files named ReadMe.txt
  
• email marked 'urgent'
  
• ?

but words sometimes (rarely?) migrate the other way, for ex. "nice" was once not something you would have wanted to be called. How does this (ie the "niceifying" of words) come about?

Added Thurs May 8: it's always nice (stupid?) to have your questions answered on the web-- niceification is called amelioration (and wall st journal tells you what you need to know)