change over vs variate

change over

verb
  • to convert to, to make a transition from one system to another 

variate

verb
  • To alter; to vary; to make or become different. 

noun
  • The measured magnitude of a variable. 

  • random variable 

  • A variable, often the set of x values plotted on a graph. 

  • A measurable quantity capable of taking on a number of values. 

How often have the words change over and variate occurred in a corpus of books? (source: Google Ngram Viewer )