fall back vs regrade

fall back

verb
  • To retreat. 

  • Used other than figuratively or idiomatically: see fall, back. 

  • To fail to fulfill a promise or purpose. 

regrade

verb
  • To regroup or reassign. 

  • To change the classification of (potentially secret documentation). 

  • To grade again, give a new grade or grading to. 

noun
  • The act or process of grading again. 

How often have the words fall back and regrade occurred in a corpus of books? (source: Google Ngram Viewer )