change over vs propel

change over

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

propel

verb
  • To provide an impetus for non-physical change, to make to arrive to a certain situation or result. 

  • To provide an impetus for motion or physical action, to cause to move in a certain direction; to drive forward. 

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